We didn't have __builtin_swap16 on all targets before GCC 4.8; hsa-brig
tries to use it if the host GCC is 4.6 or up though, breaking bootstrap.
This trivial patch fixes it.

Noticed on gcc22.


	* has-brig.c (lendian16): Don't try to use __builtin_bswap16
	unless compiling with at least GCC-4.8.

From-SVN: r235096