Switch aarch64 to __builtin_bswap16.

It generates the same code.

Bug: 18597513
Change-Id: I164296da8c676668983dd93697c6dfa05f10ec56
Switch x86 and x86-64 to __builtin_bswap*.

Bug: 18597513
Change-Id: I0781208efb3b560d7c6fa48f41be7bc6fc83521e
Merge "Switch aarch64 to __builtin_bswap16."
Merge "Switch x86 and x86-64 to __builtin_bswap*."