[flang] Use clang sysroot image to test fastmath linking

This test has been very unreliable across different machines. Update it
to use clang's sysroot image so that the fastmath object file name is
stable across different distributions and distro types.

Based on clang/test/Driver/linux-ld.c

Thanks to mnadeem for pointing this out at https://reviews.llvm.org/D138675

Differential Revision: https://reviews.llvm.org/D142807