Export cc types for art to inherit from

Art needs a custom module type in order to perform complicated
build logic like depending on environment variables and varying
cflags based on cpu variant.  Export enough of the types and
functions from cc for art_cc_library to inherit from cc_library.

While I'm touching every line, also rename the New* methods
to *Factory.

Change-Id: I7123aa47019c4ced7a1ab57c394225bc7844b5ea

cc/builder.go

@@ -88,7 +88,7 @@ type builderFlags struct {
 	ldLibs      string
 	incFlags    string
 	nocrt       bool
-	toolchain   toolchain
+	toolchain   Toolchain
 	clang       bool
 }
 
@@ -283,6 +283,6 @@ func CopyGccLib(ctx common.AndroidModuleContext, libName string,
 	})
 }
 
-func gccCmd(toolchain toolchain, cmd string) string {
+func gccCmd(toolchain Toolchain, cmd string) string {
 	return filepath.Join(toolchain.GccRoot(), "bin", toolchain.GccTriple()+"-"+cmd)
 }