Adds sys.Walltime and sys.Nanotime for security and determinism (#616)

This adds two clock interfaces: sys.Walltime and sys.Nanotime to
allow implementations to override readings for purposes of security or
determinism.

The default values of both are a fake timestamp, to avoid the sandbox
break we formerly had by returning the real time. This is similar to how
we don't inherit OS Env values.

internal/platform/mmap_test.go

@@ -11,7 +11,7 @@ import (
 var testCode, _ = io.ReadAll(io.LimitReader(rand.Reader, 8*1024))
 
 func Test_MmapCodeSegment(t *testing.T) {
-	if !IsSupported() {
+	if !CompilerSupported() {
 		t.Skip()
 	}
 
@@ -30,7 +30,7 @@ func Test_MmapCodeSegment(t *testing.T) {
 }
 
 func Test_MunmapCodeSegment(t *testing.T) {
-	if !IsSupported() {
+	if !CompilerSupported() {
 		t.Skip()
 	}