Add cflags sub-property to 'pgo'

This (possibly arch-specific) property adds extra flags while building
for profiling.  For example, dex2oat requires setting
'-Wno-frame-larger-than' when instrumentation is enabled.

Test: Build 'pgo' module with the new property and inspect build.ninja
Change-Id: Ia36422a400b0e0b87cbb33b1c8e565569475a56d
diff --git a/cc/pgo.go b/cc/pgo.go
index 9fea154..fef962e 100644
--- a/cc/pgo.go
+++ b/cc/pgo.go
@@ -42,6 +42,9 @@
 		Profile_file       *string `android:"arch_variant"`
 		Benchmarks         []string
 		Enable_profile_use *bool `android:"arch_variant"`
+		// Additional compiler flags to use when building this module
+		// for profiling (either instrumentation or sampling).
+		Cflags []string `android:"arch_variant"`
 	} `android:"arch_variant"`
 
 	PgoPresent          bool `blueprint:"mutated"`
@@ -65,6 +68,8 @@
 }
 
 func (props *PgoProperties) addProfileGatherFlags(ctx ModuleContext, flags Flags) Flags {
+	flags.CFlags = append(flags.CFlags, props.Pgo.Cflags...)
+
 	if props.isInstrumentation() {
 		flags.CFlags = append(flags.CFlags, profileInstrumentFlag)
 		// The profile runtime is added below in deps().  Add the below