add IBM f128 type

[folkertdev]

tracking issue: #161787

aka __ibm128, ppc_fp128, etc. Making this change without having settled on a final name is a bit awkward but hopefully we can get some rounds of review in anyway.

This should only merge after #160859 (so that I can deal with the merge conflicts).

I did hit that while rustc_apfloat has a DoubleDouble type, it does not implement FloatConvert (not even for conversions to itself), and its Float implementation uses Fallback, i.e. loses precision. LLVM has FIXMEs for this but clearly nobody has looked at this so far. So, a bunch of stuff for const eval / miri is stubbed out for now.

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TEST] sysroot
[COPY] sysroot_tests source
[PATCH] "sysroot_tests" <- "0027-sysroot_tests-128bit-atomic-operations.patch"
error: patch failed: coretests/tests/lib.rs:2
error: coretests/tests/lib.rs: patch does not apply
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
cd "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests" && "git" "-c" "user.name=Dummy" "-c" "user.email=dummy@example.com" "-c" "core.autocrlf=false" "-c" "commit.gpgSign=false" "am" "/checkout/compiler/rustc_codegen_cranelift/patches/0027-sysroot_tests-128bit-atomic-operations.patch" "-q" exited with status ExitStatus(unix_wait_status(32768))
Patch failed at 0001 Disable 128bit atomic operations
Bootstrap failed while executing `--stage 2 test --skip tidy --skip intrinsic-test --skip src/tools/rust-analyzer --skip tests --skip library --skip tidyselftest`
Currently active steps:
test::CodegenCranelift { compilers: RustcPrivateCompilers { build_compiler: Compiler { stage: 1, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target_compiler: Compiler { stage: 2, host: aarch64-unknown-linux-gnu, forced_compiler: false } }, target: aarch64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:4394
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout -Zembed-metadata=no --locked --color=always --profile=release --manifest-path /checkout/compiler/rustc_codegen_cranelift/build_system/Cargo.toml -- test --download-dir /checkout/obj/build/cg_clif_download --out-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif --no-unstable-features --use-backend cranelift --sysroot llvm --skip-test testsuite.extended_sysroot [workdir=/checkout/compiler/rustc_codegen_cranelift]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:4408:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:4452:26

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:24:44
  local time: Wed Sep  2 13:48:54 UTC 2026
  network time: Wed, 02 Sep 2026 13:48:55 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"