elf: Add elf/tst-dlopenfail-2 [BZ #25396]

Without CET, a jump into a newly loaded object through an overwritten
link map often does not crash, it just executes some random code.
CET detects this in some cases because the function pointer does not
point to the start of a function in the replacement shared object,
so there is no ENDBR instruction.

The new test uses a small shared object and the existing dangling
link map to trigger the bug.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>