[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

contrib/llvm-project/libcxx/include/atomic

@@ -1480,11 +1480,11 @@ struct __cxx_atomic_impl : public _Base {
     : _Base(value) {}
 };
 
-#ifdef __linux__
+#if defined(__linux__) || (defined(__FreeBSD__) && defined(__mips__))
     using __cxx_contention_t = int32_t;
 #else
     using __cxx_contention_t = int64_t;
-#endif //__linux__
+#endif
 
 #if _LIBCPP_STD_VER >= 11