Fix fabs() for MIPS when used on -0.0

[arichardson]

Summary

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.

[jhb]

My only question is if we should do the same for the arm C version.

[arichardson]

In D13135#274569, @jhb wrote:

My only question is if we should do the same for the arm C version.

https://reviews.freebsd.org/D13131

[jhb]

FreeBSD's style is to put ()'s around return values, so it should probably be 'return (__builtin_fabs(x));'

[brooks]

LGTM

[imp]

This is honestly fine as it is. Two minor nits, but they are uber nits and can be ignored perhaps...

lib/libc/mips/gen/fabs.c

43 ↗

(On Diff #35379)

The nit-pikcer's parade:
(1) should have a blank line after the { since there's no local variables.
(2) () around the return value.

These are documented in style(9), but customs are often observed in their breech rather than their adherence.