arch: set SIMDE_ARCH_ARM for AArch64 on MSVC
arch: set SIMDE_ARCH_ARM for AArch64 on MSVC

MSVC doesn't define the standard __ARM_ARCH macro.  Instead, they
have their own non-standard _M_ARM (which is defined to 7 or nothing
at all) and _M_ARM64 (which is defined to 1 or nothing at all).  I
guess they just don't support targeting 32-bit armv8.

To match the behavior of every other compiler, we define SIMDE_ARM_ARCH
to 800 when we see _M_ARM64.

This also adds code to always define SIMDE_ARCH_ARM_NEON on AArch64,
even if __ARM_NEON is not defined (MSVC doesn't define that macro,
either) since NEON is included in the AArch64 baseline.

Fixes #860.

nemequ/simde,nemequ/simde,nemequ/simde