Fix building for aarch64 windows with mingw toolchains (#555)

Fix building for aarch64 windows with mingw toolchains (#555)

* aarch64: Check _WIN32 instead of _M_ARM64 for detecting windows

This fixes building for aarch64 with mingw toolchains. _M_ARM64 is
predefined by MSVC, while mingw compilers predefine __aarch64__.

In aarch64 specific code, change checks for _M_ARM64 into checks for
_WIN32.

In arch independent code, check for
(defined(_M_ARM64) || defined(__aarch64__)) && defined(_WIN32)
instead of just _M_ARM64.

In src/closures.c, coalesce checks like
defined(X86_WIN32) || defined(X86_WIN64) || defined(_M_ARM64)
into plain defined(_WIN32). Technically, this enables code for
ARM32 windows where it wasn't, but as far as I can see it, those
codepaths should be fine for that architecture variant as well.

* aarch64: Only use armasm source when building with MSVC

When building for windows/arm64 with clang, the normal gas style .S
source works fine. sysv.S and win64_armasm.S seem to be functionally
equivalent, with only differences being due to assembler syntax.