MFC r312213,r313713:

r312213:

Turn COMPILER_VERSION/COMPILER_TYPE make check into a compile-time check
of the clang version

This works around breakage on ^/stable/10 when running installworld from
a ^/stable/10 host where the test wouldn't be compiled on the first
go-around and would be missing when make installworld is run.

PR: 208703

r313713:

Handle clang 4.x+ with the compile-time exception added in r312213

It also fails the assertions noted in bug 208703

PR: 208703
PR: 217084

MFC r292328:

Integrate a number of testcases from tools/regression/lib/msun
into the FreeBSD test suite

There's no functional change with these testcases; they're purposely
being left in TAP format for the time being

Other testcases which crash on amd64/i386 as-is have not been
integrated yet (they need to be retested on a later version of
CURRENT, as I haven't used i386 in some time)