windows: add ARM64 support

This was actually surprising once I got TinyGo to build on Windows 11
ARM64. All the changes are exactly what you'd expect for a new
architecture, there was no special weirdness just for arm64.

Actually getting TinyGo to build was kind of involved though. The very
short summary is: install arm64 versions of some pieces of software
(like golang, cmake) instead of installing them though choco. In
particular, use the llvm-mingw[1] toolchain instead of using standard
mingw.

[1]: https://github.com/mstorsjo/llvm-mingw/releases

src/runtime/asm_arm64_windows.S

@@ -0,0 +1,36 @@
+.section .text.tinygo_scanCurrentStack,"ax"
+.global tinygo_scanCurrentStack
+tinygo_scanCurrentStack:
+    // Sources:
+    //   * https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170
+    //   * https://godbolt.org/z/foc1xncvb
+
+    // Save callee-saved registers.
+    stp     x29, x30, [sp, #-160]!
+    stp     x28, x27, [sp, #16]
+    stp     x26, x25, [sp, #32]
+    stp     x24, x23, [sp, #48]
+    stp     x22, x21, [sp, #64]
+    stp     x20, x19, [sp, #80]
+    stp     d8,  d9,  [sp, #96]
+    stp     d10, d11, [sp, #112]
+    stp     d12, d13, [sp, #128]
+    stp     d14, d15, [sp, #144]
+
+    // Scan the stack.
+    mov     x0, sp
+    bl      tinygo_scanstack
+
+    // Restore stack state and return.
+    ldp     x29, x30, [sp], #160
+    ret
+
+
+.section .text.tinygo_longjmp,"ax"
+.global tinygo_longjmp
+tinygo_longjmp:
+    // Note: the code we jump to assumes x0 is set to a non-zero value if we
+    // jump from here (which is conveniently already the case).
+    ldp x1, x2, [x0] // jumpSP, jumpPC
+    mov sp, x1
+    br  x2