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: I08a054e903b202f513d3b3381afd606bb87cf257