[native] Stop linking libc++ into the CoreCLR and NativeAOT runtimes

[simonrozsival]

Now that the CoreCLR host has zero references to the C++ standard library (#12571), we can stop linking it in entirely. This is the payoff PR for the CoreCLR half of #12533.

Changes

Stop linking libc++. A new hidden coreclr-default-common CMake preset sets ANDROID_STL=none. All six coreclr-default-{debug,release}-{armeabi-v7a,arm64-v8a,x86_64} presets now inherit from it instead of the shared default-common (which MonoVM still uses).

This drops -static-libstdc++ from the link line and replaces it with -nostdlib++. The libc++ headers remain available: every CMake target already lists ${SYSROOT_CXX_INCLUDE_DIR} in its include directories, so CMake now emits those -isystem flags explicitly instead of relying on the toolchain's implicit search path.

NativeAOT gets the same treatment. NativeAOT reached zero libc++ references back in #12523, so nativeaot-default-common switches to ANDROID_STL=none as well.

Stop building libunwind for CoreCLR. add_subdirectory(common/libunwind) ran for IS_MONO_RUNTIME OR IS_CLR_RUNTIME, but only mono/tracing ever links xa::unwind. The CoreCLR host built the whole thing and never used it.

Stop shipping the archives. libc++_static.a, libc++abi.a and libunwind.a are no longer copied into the CoreCLR runtime pack, and are removed from the archive list that LinkNativeRuntime consumes. KnownSets.CplusPlusRuntime now only holds the clang built-ins archive, so it is renamed to KnownSets.CompilerRuntime ("compiler-rt").

MonoVM is untouched and keeps ANDROID_STL=c++_static.

Why this matters beyond the size win

The 316 KB the CoreCLR host lost in #12571 came from reaching zero references, at which point the linker stops pulling members out of libc++_static.a. That state is fragile: a single std::string added later silently pulls the archive back in and the win evaporates.

With -nostdlib++, that regression becomes a link error instead.

A note on LinkNativeRuntime

LinkNativeRuntime (the "link the app's native runtime statically" feature, gated on _AndroidEnableNativeRuntimeLinking) also links libcoreclr_static.a, libbrotli*.a and the BCL PAL archives from dotnet/runtime. Those are C++ and very likely still need libc++.

That property is not set anywhere in the repo today, so the feature is dormant. Removing the archives now keeps the two code paths honest about what our own runtime needs; if the feature is ever turned on, libc++ can be reintroduced there specifically rather than being linked into everything by default.

Verification

android-arm64 Release, clean CMake configures at ANDROID_STL=none:

CoreCLR

• The host compiles and links cleanly.
• libunwind does not appear in the build graph.
• libnet-android.release.so is unchanged at 203,776 bytes.
• Zero undefined libc++ / libc++abi / _Unwind_* symbols.
• DT_NEEDED unchanged; JNI_OnLoad and the Java_mono_android_Runtime_* entry points still exported.

NativeAOT

• All three artifacts -- libnaot-android.release.so, libnaot-android.release-static-release.a and libxa-java-interop-release.a -- are byte-for-byte identical (matching SHA-1) to the c++_static build.

MonoVM still builds, and still builds libunwind 1.8.1.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​native/​CMakePresets.json.in — 💡 Maintainability — coreclr-default-common duplicates ANDROID_CPP_FEATURES from…

What changed in this PR

Removes the C++ standard library (libc++/libc++abi) and libunwind from the CoreCLR native host build and runtime packs now that the host no longer references the C++ standard library, while keeping compiler built-ins available for linking.

Changes:

• Stop shipping/linking libc++/libc++abi/libunwind for CoreCLR by pruning NDK redistributables and runtime pack asset lists.
• Introduce a CoreCLR-specific CMake preset baseline (ANDROID_STL=none) and adjust CoreCLR presets to inherit from it.
• Narrow libunwind build to MonoVM-only and rename the native archive set from CplusPlusRuntime to CompilerRuntime (clang builtins).

File	Description
src/​Xamarin.Android.Build.Tasks/​Utilities/​NativeRuntimeComponents.cs	Renames the “c++” archive set to “compiler-rt” and removes libc++/libc++abi/libunwind archives from the known archive list.
src/​Xamarin.Android.Build.Tasks/​Tasks/​LinkNativeRuntime.cs	Updates the set used for the “compiler support” archive group when ordering link inputs.
src/​native/​native.targets	Stops copying NDK “CplusPlus” redistributable archives into CoreCLR runtime packs.
src/​native/​CMakePresets.json.in	Adds coreclr-default-common preset with ANDROID_STL=none and updates CoreCLR presets to inherit from it.
src/​native/​CMakeLists.txt	Builds common/libunwind only for MonoVM (where it is actually linked).
build-tools/​scripts/​Ndk.targets	Removes libc++/libc++abi/libunwind from the NDK redistributable item list.
build-tools/​scripts/​Ndk.projitems	Removes now-unused UnwindArchDir metadata.
build-tools/​create-packs/​Microsoft.Android.Runtime.proj	Stops including NDK “CplusPlus” redistributables in the CoreCLR runtime pack assets.