lib/crypto: arm64/sha512-ce: Drop compatibility macros for older binutils

lib/crypto: arm64/sha512-ce: Drop compatibility macros for older binutils

Now that the oldest supported binutils version is 2.30, the macros that
emit the SHA-512 instructions as '.inst' words are no longer needed.  So
drop them.  No change in the generated machine code.

Changed from the original patch by Ard Biesheuvel:
(https://lore.kernel.org/r/20250515142702.2592942-2-ardb+git@google.com):
 - Reduced scope to just SHA-512
 - Added comment that explains why "sha3" is used instead of "sha2"

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250718220706.475240-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>