[flang-rt][ppc64le] Build fail: FLANG_RUNTIME_F128_MATH_LIB setting requires quadmath.h to be available

## Description

On ppc64le, `libquadmath-devel` installs `quadmath.h` only to GCC's private include directory (e.g. `/usr/lib/gcc/ppc64le-redhat linux/14/include/quadmath.h`), not to `/usr/include/`. Same thing on Ubuntu. The just-built clang cannot find this header when trying to build `flang-rt`. And AFAIK `libquadmath` is not needed because ppc64le natively supports IBM double-double and IEEE quad.

## Steps to Reproduce

On a ppc64le (POWER10 in my case) system:

```bash
cmake -S llvm -B build -G Ninja \
-DLLVM_ENABLE_PROJECTS="clang;flang" \
-DLLVM_ENABLE_RUNTIMES="flang-rt" \
-DLLVM_TARGETS_TO_BUILD="PowerPC" \
-DCMAKE_BUILD_TYPE=Release
ninja -C build flang-rt

Error

-- Found GCC installation selected by Clang: /usr/lib/gcc/ppc64le-redhat-linux/14
-- Looking for quadmath.h
-- Looking for quadmath.h - not found
CMake Error at flang-rt/lib/quadmath/CMakeLists.txt:84 (message):
FLANG_RUNTIME_F128_MATH_LIB setting requires quadmath.h to be available: libquadmath
```

## Workaround

```
if(CMAKE_SYSTEM_PROCESSOR MATCHES "ppc64le")
set(FLANG_RUNTIME_F128_MATH_LIB "")
else()
set(FLANG_RUNTIME_F128_MATH_LIB "libquadmath")
endif()
```

Environment

- POWER10
- Tested on: AlmaLinux 10.2 (GCC 14), Ubuntu 24.04 (GCC 13)

Start at flang-rt/lib/quadmath/CMakeLists.txt around line 84 and reproduce the failure with the provided CMake and Ninja commands on ppc64le. Inspect how FLANG_RUNTIME_F128_MATH_LIB and quadmath.h availability are handled, including the reported GCC private include path and workaround. Done means flang-rt configures and builds successfully on the reported ppc64le environments.

Written by the indexing model from the issue text.