Specifically check for NEON for ARMv8 CPUs

[dm0-]

The actual NEON implementation uses instructions that are not supported on 32-bit CPUs. These errors happen when building for armv7a:

/var/tmp/portage/app-arch/snappy-1.1.10-r1/work/snappy-1.1.10/snappy-internal.h:106:10: error: ‘vminvq_u8’ was not declared in this scope; did you mean ‘vminq_u8’?
  106 |   assert(vminvq_u8(shuffle_mask) >= 0 && vmaxvq_u8(shuffle_mask) <= 15);
      |          ^~~~~~~~~
/var/tmp/portage/app-arch/snappy-1.1.10-r1/work/snappy-1.1.10/snappy-internal.h:106:42: error: ‘vmaxvq_u8’ was not declared in this scope; did you mean ‘vmaxq_u8’?
  106 |   assert(vminvq_u8(shuffle_mask) >= 0 && vmaxvq_u8(shuffle_mask) <= 15);
      |                                          ^~~~~~~~~
/var/tmp/portage/app-arch/snappy-1.1.10-r1/work/snappy-1.1.10/snappy-internal.h:107:10: error: ‘vqtbl1q_u8’ was not declared in this scope; did you mean ‘vtbl1_u8’?
  107 |   return vqtbl1q_u8(input, shuffle_mask);
      |          ^~~~~~~~~~
      |          vtbl1_u8

This PR just makes the CMake test use the vmaxvq_u8 intrinsic to find the maximum vector element to test for A64 NEON, since that's an instruction used in the code. You can check supported instruction targets here: https://arm-software.github.io/acle/neon_intrinsics/advsimd.html

[danilak-G]

Fixed with 32ded45, it is a slightly better fix, thanks for reporting!