[compiler-rt] CompilerRT: Normalize COMPILER_RT_DEFAULT_TARGET_TRIPLE (PR #88835)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Thanks for digging Martin, I was pinged about this too today.

This also broke libcxx Picolib checks at the install step. This may be because this build does not use the per target runtime dir at all, being an embedded build, not sure yet.

https://buildkite.com/llvm-project/libcxx-ci/builds/34897#018eec06-612c-47f1-9931-d3bd88bf7ced
```
+ mv '/home/david.spickett/llvm-project/build/armv7m-picolibc/install/lib/armv7m-none-eabi/*' /home/david.spickett/llvm-project/build/armv7m-picolibc/install/lib
mv: cannot stat '/home/david.spickett/llvm-project/build/armv7m-picolibc/install/lib/armv7m-none-eabi/*': No such file or directory
```
To reproduce this you'll need qemu-system-arm 8.1.3 or newer installed, set CC/CXX to clang 17/18 (latest is ideal) and then from the llvm-project dir run `./libcxx/utils/ci/run-buildbot armv7m-picolibc`. If it gets to the test stage it's got past the install step.

The triple ends up converted to thumbv7:
```
-- cmake c compiler target: armv7m-none-eabi
-- armv7m-none-eabi
CMake Error at cmake/Modules/CompilerRTUtils.cmake:378 (message):
  thumbv7m-none-unknown-eabi
Call Stack (most recent call first):
```

And I don't see any sign of either triple in any folder names in the `/lib`.

https://github.com/llvm/llvm-project/pull/88835