Significantly reduce compile time for googletest internal tests

Clang's optimizer spends a really long time on these tests at -O2, so we now
use -O0 instead. This reduces the -j32 time for lib/googletest/test from 131s
to 29s. Using -O0 also reduces the disk usage from 144MB (at -O2) / 92MB (at
-O1) to 82MB.

Reviewed By: ngie, dim
Differential Revision: https://reviews.freebsd.org/D26751