Instead of allowing the Crypto Extensions algorithms to be selected when
using a toolchain that does not support them, and complain about it at
build time, use the information we have about the compiler to prevent
them from being selected in the first place. Users that are stuck with
a GCC version <4.8 are unlikely to care about these routines anyway, and
it cleans up the Makefile considerably.

While at it, add explicit 'armv8-a' CPU specifiers to the code that uses
the 'crypto-neon-fp-armv8' FPU specifier so we don't regress Clang, which
will complain about this in version 10 and later.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>