Let genrule be mutatable by the apex mutator

genrule modules were not mutated by the apex mutator. As a result, a
module that is mutated for an apex have depended on the genrule modules
host_bionic_linker_{asm|flags} that are not mutated for the APEX.

This in turn caused an inconsistency problem because the
host_bionic_linker_* genrules use the dynamic linker that isn't mutated
for APEX as an input, while the outputs of the genrules are used by
modules mutated for APEX. The inconsistency caused b/135008828.

Fixing this issue by correctly let genrule to be mutated by the apex
mutator.

Bug: 135008828
Test: checkout master-art-host
ALLOW_MISSING_DEPENDENCIES=true DIST_DIR=out/dist
./art/tools/dist_linux_bionic.sh -j50 showcommands
com.android.runtime.host
is passing

Change-Id: I92d90400c61d3951f6202d932832d0cb63c95a7d

cc/genrule.go

@@ -42,5 +42,7 @@ func genRuleFactory() android.Module {
 
 	android.InitAndroidArchModule(module, android.HostAndDeviceSupported, android.MultilibBoth)
 
+	android.InitApexModule(module)
+
 	return module
 }

genrule/genrule.go

@@ -101,6 +101,7 @@ type generatorProperties struct {
 type Module struct {
 	android.ModuleBase
 	android.DefaultableModuleBase
+	android.ApexModuleBase
 
 	// For other packages to make their own genrules with extra
 	// properties