crypto: aegis128-neon - use Clang compatible cflags for ARM

The next version of Clang will start policing compiler command line
options, and will reject combinations of -march and -mfpu that it
thinks are incompatible.

This results in errors like

  clang-10: warning: ignoring extension 'crypto' because the 'armv7-a'
  architecture does not support it [-Winvalid-command-line-argument]
  /tmp/aegis128-neon-inner-5ee428.s: Assembler messages:
            /tmp/aegis128-neon-inner-5ee428.s:73: Error: selected
  processor does not support `aese.8 q2,q14' in ARM mode

when buiding the SIMD aegis128 code for 32-bit ARM, given that the
'armv7-a' -march argument is considered to be compatible with the
ARM crypto extensions. Instead, we should use armv8-a, which does
allow the crypto extensions to be enabled.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>