[flang] Fix linker test on Windows.

The linker-flags.f90 test checks for the linker command line. The `-target` indicates cross-compiling, the toolchain executables themselves are still running on the native platform. If it is Windows, the driver will try to fully resolve the path to `ld` which may include an `.exe` suffix.

In my case, it resolves to the MinGW installation (`"C:\\tools\\msys64\\usr\\bin\\ld.exe"`) found in `PATH`. The GNU ld that comes with the MSYS2 distribution does not support `elf64lppc` or MacOS emulation modes (`acosx_version_min`), but the test also does not require executing the linker.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D144592