Fix openmp-offload-amdgpu-clang-flang

[Meinersbur]

Since 0ac8f9d, openmp-offload-amdgpu-clang-flang is failing with

CMake Error at /home/botworker/builds/openmp-offload-amdgpu-clang-flang/llvm.src/flang-rt/lib/quadmath/CMakeLists.txt:85 (message):
  FLANG_RUNTIME_F128_MATH_LIB setting requires quadmath.h to be available:
  libquadmath

The reason is that flang-rt now is built using Clang (like all LLVM runtimes in a bootstrapping build), instead of gcc. quadmath.h is located in gcc's "resource dir" (on my system, it is /usr/lib/gcc/x86_64-linux-gnu/13/include/quadmath.h) instead in default paths such as /usr/include. Hence, clang cannot find it.

Remove the FLANG_RUNTIME_F128_MATH_LIB=libquadmath option since by design it does not work with Clang.

Alterantive approaches:

• Instead of a bootstrapping build, do a out-of-tree build of flang-rt where we can set CMAKE_CXX_COMPILER=gcc.
• Clang already looks for compatible gcc installation to access its libc files such as crtbegin.o. It wouldn't be too far fetched to also add its include dir to Clang's default search path. Otherwise, we need to wait until LLVM gets its own equivalent of libquadmath.

[jplehr]

Adding @dpalermo for awareness.

[jplehr]

LGTM