clang: use -mxgot for 32-bit mips

Various bits in usr.bin/clang/* will fail to compile without -mxgot due to
truncated relocations. -mxgot entails a speed penalty, but I suspect we
don't care as much about compiler performance in 32-bit mips land.

Reviewed by: arichardson
Differential Revision: https://reviews.freebsd.org/D21698