Add further test of TLS

Add further test of TLS

Add an additional test of TLS variables, with different alignment,
accessed from different modules.  The idea of the alignment test is
similar to tst-tlsalign and the same code is shared for setting up
test variables, but unlike the tst-tlsalign code, there are multiple
threads and variables are accessed from multiple objects to verify
that they get a consistent notion of the address of an object within a
thread.  Threads are repeatedly created and shut down to verify proper
initialization in each new thread.  The test is also repeated with TLS
descriptors when supported.  (However, only initial-exec TLS is
covered in this test.)

Tested for x86_64.