Prepare for powerpc-on-clang by deleting hard-coded tests for libstdc++.so
as a stand-in for "are we running on gcc".

For people already testing powerpc on clang, it is possible that they
already have both compilers in base. Thus, the assumption that "gcc is
in base" (e.g. libstdc++.so exists) always means "force use of GCC" is
already broken. It will be for everyone on -CURRENT once the switch is
made.

Tested on both amd64 and powerpc64.

PR: 239153
Approved by: gnome (maintainer-timeout, > 1 month)