MFC r292000: Remove historical GNUC test

MFC r292000: Remove historical GNUC test

The requirement is for a GCC-compatible compiler and not necessarily
GCC itself. However, we currently expect any compiler used for building
the whole of FreeBSD to be GCC-compatible and many things will break if
not; there's no longer a need to have an explicit test for this in csu.

git-svn-id: svn://svn.freebsd.org/base/stable/10@300323 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f