Use the FreeBSD __CONCAT.

We had the NetBSD __CONCAT macro, despite the fact that our only callers
are FreeBSD code. The FreeBSD libm changes in FreeBSD r336665 require
the FreeBSD implementation rather than the NetBSD one.

Unfortunately for me, that libm code is built in the NDK build too,
where it's built against the NDK's <sys/cdefs.h>. So I need to update
__CONCAT first before I can update the libm code.

Bug: http://b/111710419
Test: ran tests
Change-Id: I70e76e4caab2929c939bfb7009df9fe79d26806a