runtime: implement race-free signals using futexes

This requires an API introduced in MacOS 11. I think that's fine, since
the version before that (MacOS 10.15) is EOL since 2022. Though if
needed, we could certainly work around it by using an older and slightly
less nice API.

loader/goroot.go

@@ -243,6 +243,7 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		"internal/binary/":            false,
 		"internal/bytealg/":           false,
 		"internal/cm/":                false,
+		"internal/futex/":             false,
 		"internal/fuzz/":              false,
 		"internal/reflectlite/":       false,
 		"internal/gclayout":           false,