[BTI] Add new unit tests.

Add tests for the linker and shared libraries.

These new tests call a simple function from a shared library
which has the BTI note. Function calls should work as before
if the function starts with a landing pad. Bypassing this landing pad
should cause SIGILL.

Change-Id: I6cc731ee4800af6f89d3d1d866b216e0f4832a94