UWP/MSVC support and some misc fixes

commit 6fec0c20:

meson: Fix ASM visibility attribute for Darwin

commit e3616c32:

meson: Disable some unused mmap and alloca checks

commit e22d3c04:

meson: Fix some MSVC assembly feature checks
These should really be configure checks, but they require inspecting
the compiled output, which we can't with meson.

commit 539d9856:

aarch64: Fix some MSVC compiler warnings with explicit casts

commit 94dbfe98:

aarch64: Add support for the arm64 WinRT target
Code copied from https://github.com/libffi/libffi/pull/486, cleaned
up and split into separate commits with Meson support.

Author is ossdev07 <ossdev@puresoftware.com>