Remove execute-only support.

With kernel support being removed for execute-only memory layouts,
remove support for XOM until there's kernel support again.

This allows us to gain the code-size improvements from
"-z noseparate-code".

Bug: 147300048
Test: Compiled binaries no longer marked XOM by default.
Change-Id: Ie84bdd2ca025e8b14867008291bf9ce143b5e999

Android.bp

@@ -195,7 +195,6 @@ bootstrap_go_package {
         "cc/vendor_snapshot.go",
         "cc/vndk.go",
         "cc/vndk_prebuilt.go",
-        "cc/xom.go",
 
         "cc/cflag_artifacts.go",
         "cc/cmakelists.go",