[libc++] Fix locale-related compilation errors on NetBSD (#143055)
To my knowledge, NetBSD is mostly like other BSDs, but doesn't have
`xlocale.h`. I think c664a7f may have inadvertently broken this.
With this change, I was able to run
[zig-bootstrap](https://github.com/ziglang/zig-bootstrap) to completion
for `x86_64-netbsd10.1-none`.