Move bionic-unit-tests-static to libclang_rt.builtins

Bug: http://b/129728267
Bug: http://b/117167374

This test is now built with XOM (because libm now uses XOM) but it
should also use libclang_rt.builtins since libgcc is not built with XOM.

Test: bionic-unit-tests-static passes (but used to segfault with XOM
exceptions without this change).

Change-Id: I0b1dfe52e48db5f5e9386c7722ecd91e155be56f