Remove tests for obsolete compilers in the build system

Remove tests for obsolete compilers in the build system

Assume gcc is at least 6.4, the oldest xtoolchain in the ports tree.
Assume clang is at least 6, which was in 11.2-RELEASE.  Drop conditions
for older compilers.

Reviewed by:	imp (earlier version), emaste, jhb
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D24802

(cherry picked from commit fac6dee9eb58b2b558fec2aea749460ca623f6d6)