[ARM] Improve diagnostics message when Neon is unsupported

Summary:
Whenever Neon is not supported, a generic message is printed:

  error: "NEON support not enabled"

Followed by a series of other error messages that are not useful once
the first one is printed.

This patch gives a more precise message in the case where Neon is
unsupported because an invalid float ABI was specified: the soft float
ABI.

  error: "NEON intrinsics not available with the soft-float ABI. Please
  use -mfloat-abi=softfp or -mfloat-abi=hard"

This message is the same one that GCC gives, so it is also making their
diagnostics more compatible with each other.

Also, by rearranging preprocessor directives, these "unsupported" error
messages are now the only ones printed out, which is also GCC's
behaviour.

Differential Revision: https://reviews.llvm.org/D81847