__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 not defined on aarch64

[Toebs Douglass]

Hej all.

__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 is defined on x86_64 if and only if
-mcx16 is given.

gcc version 4.9.2 (Debian 4.9.2-10)
winterflaw@localhost:~$ gcc -march=native -dM -E - < /dev/null | sort |
grep SYNC
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1

__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 is not defined at all on aarch64, as
far as I can tell.

gcc version 4.9.2 (Debian/Linaro 4.9.2-10)
debian@pine64:~$ gcc -march=armv8-a+crypto+crc -dM -E - < /dev/null |
sort | grep SYNC
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1

Both are the OS provided GCCs (Debian 8 in both cases).