aarch64: use __ARM_NEON instead of __aarch64__ to control use of neon

aarch64: use __ARM_NEON instead of __aarch64__ to control use of neon

There are compilation environments in aarch64 where NEON isn't
available. While these environments could define ZSTD_NO_INTRINSICS,
it's more fail-safe to use the more specific symbol to know if NEON
extensions are available.

__ARM_NEON is the proper symbol, defined in ARM C Language Extensions
Release 2.1 (https://developer.arm.com/documentation/ihi0053/d/). Some
sources suggest __ARM_NEON__, but that's the obsolete spelling from
prior versions of the standard.

Signed-off-by: Warner Losh <imp@bsdimp.com>