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

compileopts/target.go

@@ -304,10 +304,19 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		// normally present in Go (without explicitly opting in).
 		// For more discussion:
 		// https://groups.google.com/g/Golang-nuts/c/Jd9tlNc6jUE/m/Zo-7zIP_m3MJ?pli=1
+		switch goarch {
+		case "amd64":
+			spec.LDFlags = append(spec.LDFlags,
+				"-m", "i386pep",
+				"--image-base", "0x400000",
+			)
+		case "arm64":
+			spec.LDFlags = append(spec.LDFlags,
+				"-m", "arm64pe",
+			)
+		}
 		spec.LDFlags = append(spec.LDFlags,
-			"-m", "i386pep",
 			"-Bdynamic",
-			"--image-base", "0x400000",
 			"--gc-sections",
 			"--no-insert-timestamp",
 			"--no-dynamicbase",