Fix fabs() for MIPS when used on -0.0

It would previously return negative zero for -0.0 since -0.0 does not
compare less than 0. The issue was discovered when running the libc++
test suite on softfloat MIPS64.

I have verified that both clang and GCC generate sensible code for the
builtin. For soft float they clear the sign bit using integer operations
and in hard float mode they use abs.d.

Reviewed by: MIPS, jhb, brooks, imp, emaste
Approved by: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D13135