Support per-module MakeVars

This allows setting per-module make variables earlier in the build,
particularly for prebuilt_build_tool users like LEX/M4/BISON. I moved
filegroup over because it's a simpler common interface, but it doesn't
strictly need it.

With this, the last user of the hardcoded cc.m4Cmd variable is gone.

Test: Inspect out/soong/make_vars-*.mk, out/soong/late-*.mk
Test: treehugger
Change-Id: I195b688131feac0c100c338a0749368aa5d50f4f

cc/gen.go

@@ -24,8 +24,6 @@ import (
 )
 
 func init() {
-	pctx.SourcePathVariable("m4Cmd", "prebuilts/build-tools/${config.HostPrebuiltTag}/bin/m4")
-
 	pctx.HostBinToolVariable("aidlCmd", "aidl-cpp")
 	pctx.HostBinToolVariable("syspropCmd", "sysprop_cpp")
 }