MSVC: Support building for AArch64.

MSVC: Support building for AArch64.

This commit does the following to get tests passing for
MSVC/AArch64:

* Implements spin_delay() with an ISB instruction (like we do for
gcc/clang on AArch64).

* Sets USE_ARMV8_CRC32C unconditionally.  Vendor-supported versions
of Windows for AArch64 require at least ARMv8.1, which is where CRC
extension support became mandatory.

* Implements S_UNLOCK() with _InterlockedExchange().  The existing
implementation for MSVC uses _ReadWriteBarrier() (a compiler
barrier), which is insufficient for this purpose on non-TSO
architectures.

There are likely other changes required to take full advantage of
the hardware (e.g., atomics/arch-arm.h, simd.h,
pg_popcount_aarch64.c), but those can be dealt with later.

Author: Niyas Sait <niyas.sait@linaro.org>
Co-authored-by: Greg Burd <greg@burd.me>
Co-authored-by: Dave Cramer <davecramer@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Tested-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/A6152C7C-F5E3-4958-8F8E-7692D259FF2F%40greg.burd.me
Discussion: https://postgr.es/m/CAFPTBD-74%2BAEuN9n7caJ0YUnW5A0r-KBX8rYoEJWqFPgLKpzdg%40mail.gmail.com