[cc/pgo] Mark pgo.sampling property as an arch-variant

Bug: http://b/189438896

This allows disabling sampling PGO for some arches.

Test: Build this topic.
Change-Id: Ib6ddca6685c2e53092afd440ad95ec42d3a9538b
Merged-In: Ib6ddca6685c2e53092afd440ad95ec42d3a9538b
(cherry picked from commit 19a5aae35e)

cc/pgo.go

@@ -56,7 +56,7 @@ func recordMissingProfileFile(ctx BaseModuleContext, missing string) {
 type PgoProperties struct {
 	Pgo struct {
 		Instrumentation    *bool
-		Sampling           *bool
+		Sampling           *bool   `android:"arch_variant"`
 		Profile_file       *string `android:"arch_variant"`
 		Benchmarks         []string
 		Enable_profile_use *bool `android:"arch_variant"`