internal/cpu: add invalid option warnings and support to enable cpu features

This CL adds the ability to enable the cpu feature FEATURE by specifying
FEATURE=on in GODEBUGCPU. Syntax support to enable cpu features is useful
in combination with a preceeding all=off to disable all but some specific
cpu features. Example:

GODEBUGCPU=all=off,sse3=on

This CL implements printing of warnings for invalid GODEBUGCPU settings:
- requests enabling features that are not supported with the current CPU
- specifying values different than 'on' or 'off' for a feature
- settings for unkown cpu feature names

Updates #27218

Change-Id: Ic13e5c4c35426a390c50eaa4bd2a408ef2ee21be
Reviewed-on: https://go-review.googlesource.com/c/141800
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/internal/cpu/cpu_test.go

@@ -30,7 +30,10 @@ func runDebugOptionsTest(t *testing.T, test string, options string) {
 	cmd.Env = append(cmd.Env, env)
 
 	output, err := cmd.CombinedOutput()
-	got := strings.TrimSpace(string(output))
+	lines := strings.Fields(string(output))
+	lastline := lines[len(lines)-1]
+
+	got := strings.TrimSpace(lastline)
 	want := "PASS"
 	if err != nil || got != want {
 		t.Fatalf("%s with %s: want %s, got %v", test, env, want, got)