[libcxx] Fix atomic type for mips32 on gcc to work w/out needing libatomic

When compiling this for mips32 on gcc-6.x, we'd hit issues where we
don't have 64 bit atomics on mips32.

gcc implements this using libatomic, which we don't currently include
in our freebsd-gcc compiler packages.

So for now add this work around so mips32 works. It's also fine for
mips64. We can fix this later once we get libatomic included.

Approved by: dim
Differential Revision: https://reviews.freebsd.org/D26774