configure.ac: Introduce HAVE_ARM_ASM/HAVE_AARCH64_ASM and the -D flags.

configure.ac: Introduce HAVE_ARM_ASM/HAVE_AARCH64_ASM and the -D flags.

I've been trying to get away without these conditionals in vc4's NEON
code, but it meant compiling extra unused code on x86, and build failing
on ARMv6.

v2: Use the _arm/_arm64 flags to simplify detection (suggested by Rob),
    but hide the _arm version under ARCH_ARM_HAVE_NEON to keep from trying
    to build this stuff for armv5te.

Tested-by: Rob Herring <robh@kernel.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>