Add support for AArch64 XOM binaries.

Adds build system support for generating AArch64 binaries with
execute-only memory layouts via a new LOCAL_XOM property. Also adds
support for an ENABLE_XOM build flag for global builds.

Bug: 77958880
Test: make -j ENABLE_XOM=true
Change-Id: I6af9e3615d0a9fdff802eae50e6ad94311ec8046