[clang] [MinGW] Don't look for a GCC in path if the install base has a proper...

[clang] [MinGW] Don't look for a GCC in path if the install base has a proper mingw sysroot (#76949)

This fixes uses of the MSYS2 clang64 environment compilers, if another
set of GCC based compilers are available further back in PATH (which may
be explicitly added, or inherited unintentionally from other software
installed).

(The issue in the clang64 environment can be worked around somewhat by
installing *-gcc-compat packages which present aliases named
<triple>-gcc within the clang64 environment as well.)

This fixes https://github.com/msys2/MINGW-packages/issues/11495 and
https://github.com/msys2/MINGW-packages/issues/19279.