CompilerRT: Normalize COMPILER_RT_DEFAULT_TARGET_TRIPLE (#89234)

If LLVM is configured with -DLLVM_DEFAULT_TARGET_TRIPLE, or compiler_rt
is configured with -DCOMPILER_RT_DEFAULT_TARGET_TRIPLE, while the
argument is not normalized, such as Debian-style vendor-less triple,
clang will try to find libclang_rt in lib/<normalized_triple>, while
libclang_rt is placed into lib/<triple_arg>.

Let's also place libclang_rt into lib/<normalized_triple>.
`libcxx/utils/ci/run-buildbot` is also updated to use
`armv7m-none-unknown-eabi` as normalized triple instead of
current `armv7m-none-eabi`.

ChangeLog of this PR:
This patch has been applied and revert twice:
1. The first try is #88407, and
then it is found that it causes some CI failures.
    https://lab.llvm.org/buildbot/#/builders/98/builds/36366
It is then resolved by another commit:
1693009

    https://lab.llvm.org/buildbot/#/builders/77/builds/36372
It is caused that `COMPILER_RT_DEFAULT_TARGET_TRIPLE` is overwrite
without taking care about `CACHE`.

2. The second try #88835,
     resolves https://lab.llvm.org/buildbot/#/builders/77/builds/36372
     and in fact only one `execute_process` is needed.

Then we find some other CI failures. 
https://github.com/mstorsjo/llvm-mingw/actions/runs/8730621159

https://buildkite.com/llvm-project/libcxx-ci/builds/34897#018eec06-612c-47f1-9931-d3bd88bf7ced

It is due to misunderstanding `-print-effective-triple`: which will
output `thumbv7-w64-windows-gnu` for `armv7-w64-windows-gnu` or some
other thumb-enabled arm triples.
In fact we should use `-print-target-triple`.

For armv7m-picolibc, `armv7m-none-eabi` is hardcoded in
libcxx/utils/ci/run-buildbot, while in fact `armv7m-none-unknown-eabi`
is the real normalized triple.

compiler-rt/cmake/Modules/CompilerRTUtils.cmake

@@ -368,6 +368,12 @@ macro(construct_compiler_rt_default_triple)
           "Default triple for which compiler-rt runtimes will be built.")
   endif()
 
+  if ("${CMAKE_C_COMPILER_ID}" MATCHES "Clang")
+    execute_process(COMMAND ${CMAKE_C_COMPILER} --target=${COMPILER_RT_DEFAULT_TARGET_TRIPLE} -print-target-triple
+                    OUTPUT_VARIABLE COMPILER_RT_DEFAULT_TARGET_TRIPLE
+                    OUTPUT_STRIP_TRAILING_WHITESPACE)
+  endif()
+
   string(REPLACE "-" ";" LLVM_TARGET_TRIPLE_LIST ${COMPILER_RT_DEFAULT_TARGET_TRIPLE})
   list(GET LLVM_TARGET_TRIPLE_LIST 0 COMPILER_RT_DEFAULT_TARGET_ARCH)
 

libcxx/utils/ci/run-buildbot

@@ -217,7 +217,7 @@ function test-armv7m-picolibc() {
         "${@}"
 
     ${NINJA} -vC "${BUILD_DIR}/compiler-rt" install
-    mv "${BUILD_DIR}/install/lib/armv7m-none-eabi"/* "${BUILD_DIR}/install/lib"
+    mv "${BUILD_DIR}/install/lib/armv7m-none-unknown-eabi"/* "${BUILD_DIR}/install/lib"
 
     check-runtimes
 }