gdb/testsuite/gdb.threads: Ensure TLS tests link against pthreads.

On Ubuntu by default the compiler passes --as-needed to ld which
means no DT_NEEDED entry is added for libpthread when building
the TLS tests. This causes the test to fail as libpthread is
required to look up TLS variables. Add calls to pthread_testcancel
to make sure libpthread gets linked.

gdb/testsuite/ChangeLog:

2013-07-18  Will Newton  <will.newton@linaro.org>

	* gdb.threads/tls-nodebug.c: Call pthread_testcancel
	to ensure the test is linked against pthreads.
	* gdb.threads/tls-var-main.c: Likewise.
	* gdb.threads/tls-shared.c: Likewise.