New test - gdb.base/tls-multiobj.exp

New test - gdb.base/tls-multiobj.exp

This test exercises GDB's internal TLS support when both the main
program and several shared libraries have TLS variables.  It also
tests existing (non-internal) TLS support too.

It tests using two compilation scenarios, "default", in which
libpthread is not linked with the program and libraries as well
as one which does use libpthread.

It tests link map address to module id mapping code in GDB
in addition to the ability of GDB to traverse TLS data structures
with several libraries in play.

Tested-By: Luis Machado <luis.machado@arm.com>
Approved-By: Luis Machado <luis.machado@arm.com>