builder: link stack probes on Windows amd64 and arm64

Go 1.27 packages can emit stack probes on Windows amd64 and arm64.
Link the compiler-rt stack probe builtins and use compiler-rt for those
targets so these packages build.

builder/builtins.go

@@ -207,6 +207,16 @@ var windowsI386Builtins = []string{
 	"i386/chkstk.S", // also _alloca
 }
 
+// Builtins needed specifically for windows/amd64.
+var windowsAMD64Builtins = []string{
+	"x86_64/chkstk.S",
+}
+
+// Builtins needed specifically for windows/arm64.
+var windowsARM64Builtins = []string{
+	"aarch64/chkstk.S",
+}
+
 // libCompilerRT is a library with symbols required by programs compiled with
 // LLVM. These symbols are for operations that cannot be emitted with a single
 // instruction or a short sequence of instructions for that target.
@@ -233,13 +243,28 @@ var libCompilerRT = Library{
 			builtins = append(builtins, aeabiBuiltins...)
 		case "avr":
 			builtins = append(builtins, avrBuiltins...)
-		case "x86_64", "aarch64", "riscv64": // any 64-bit arch
+		case "x86_64":
+			builtins = append(builtins, genericBuiltins128...)
+			if isWindowsTriple(target) {
+				builtins = append(builtins, windowsAMD64Builtins...)
+			}
+		case "aarch64":
+			builtins = append(builtins, genericBuiltins128...)
+			if isWindowsTriple(target) {
+				builtins = append(builtins, windowsARM64Builtins...)
+			}
+		case "riscv64":
 			builtins = append(builtins, genericBuiltins128...)
 		case "i386":
-			if strings.Split(target, "-")[2] == "windows" {
+			if isWindowsTriple(target) {
 				builtins = append(builtins, windowsI386Builtins...)
 			}
 		}
 		return builtins, nil
 	},
 }
+
+func isWindowsTriple(target string) bool {
+	parts := strings.Split(target, "-")
+	return len(parts) > 2 && parts[2] == "windows"
+}

compileopts/target.go

@@ -475,10 +475,12 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 				"-m", "i386pep",
 				"--image-base", "0x400000",
 			)
+			spec.RTLib = "compiler-rt"
 		case "arm64":
 			spec.LDFlags = append(spec.LDFlags,
 				"-m", "arm64pe",
 			)
+			spec.RTLib = "compiler-rt"
 		}
 		spec.LDFlags = append(spec.LDFlags,
 			"-Bdynamic",