testsuite: Do not restrict stack protector tests to native testing

testsuite: Do not restrict stack protector tests to native testing

Three stack protector tests in gcc.dg use

/* { dg-do run { target native } } */

for no apparent reason, since there is nothing in those tests that
should care about the difference between native and cross testing.
They also have

/* { dg-require-effective-target fstack_protector } */

which is the actually relevant condition for such tests.  Remove the
{ target native } restriction.

Tested natively for x86_64-pc-linux-gnu, and with cross to
aarch64-linux.

	* gcc.dg/ssp-1.c, gcc.dg/ssp-2.c, gcc.dg/stackprotectexplicit1.c:
	Do not restrict to { target native }.