Add support for Windows on aarch64

[HertzDevil]

Adds ARM64-specific stack unwinding so that the prelude now compiles for Windows on ARM64.

This is tested in a UTM virtual machine only. To set up:

• Install the MSVC build tools with ARM64/ARM64EC support (we won't be dealing with ARM64EC at all);
• Install Crystal 1.13. Note that Windows ARM64 can run x86-64 binaries directly;
• Check out this branch;
• Start an MSVC native build tools shell, enter PowerShell, and then build Boehm GC and PCRE2 with the following:

  .\etc\win-ci\build-gc.ps1 -BuildTree deps\gc -Version 8.2.6 -AtomicOpsVersion 7.8.2
  .\etc\win-ci\build-gc.ps1 -BuildTree deps-md\gc -Version 8.2.6 -AtomicOpsVersion 7.8.2 -Dynamic
  .\etc\win-ci\build-pcre2.ps1 -BuildTree deps\pcre2 -Version 10.43
  .\etc\win-ci\build-pcre2.ps1 -BuildTree deps-md\pcre2 -Version 10.43 -Dynamic

• Override the CRYSTAL_LIBRARY_PATH environment variable so that it points at the libs directory created from the previous step;
• Use bin\crystal build --target=aarch64-pc-windows-msvc --target -Dwithout_iconv to build something; (we don't need to supply --cross-compile or build a local compiler here)
• Copy the DLLs inside the dlls directory created above to the current directory manually, since the ones placed by the compiler are the x86-64 ones;
• You should now have a fully working Windows ARM64 executable built by Crystal.

For convenience, the x86-64 and ARM64 Windows bindings share the same directory using a symbolic link.

The context switch reuses the existing one for AArch64 Linux, since it seems the ABIs are identical; most Fiber and select specs work, except ones that deal with closed Channels for some reason.

[straight-shoota]

note: This const value is a neat gimmick. 😆