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