require tls_runtime for tls execution test

require tls_runtime for tls execution test

All TLS execution tests require tls_runtime, not just tls; pr78796.c
is the only exception that is not otherwise limited to platforms known
to support it.  I suppose that's an oversight.  On a platform whose
linker is configured to disregard TLS relocations, this test compiles
and assembles successfully, but execution fails.  The tls_runtime
requirement target avoids the noise from the expected failure.

for  gcc/testsuite/ChangeLog

	* gcc.dg/tls/pr78796.c: Require tls_runtime.