Fix openmp-offload-amdgpu-clang-flang (llvm#395)

Since 0ac8f9d,
[openmp-offload-amdgpu-clang-flang](https://lab.llvm.org/staging/#/builders/105/builds/16994)
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.

Author: Meinersbur

buildbot/osuosl/master/config/builders.py

@@ -2043,7 +2043,6 @@
                             "-DLLVM_ENABLE_ASSERTIONS=ON",
                             "-DCMAKE_C_COMPILER_LAUNCHER=ccache",
                             "-DCMAKE_CXX_COMPILER_LAUNCHER=ccache",
-                            "-DFLANG_RUNTIME_F128_MATH_LIB=libquadmath",
                             "-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON",
                             "-DCMAKE_CXX_STANDARD=17",
                             "-DBUILD_SHARED_LIBS=ON",