testsuite - improve check_effective_target_cet.

testsuite - improve check_effective_target_cet.

In some cases the test using setssbsy was not enough to
detemine support for the CET insns.  Adding -fcf-protection
explicitly causes other insns to be emitted (e.g. endbr32/64)
which are a more complete check.

2019-05-16  Iain Sandoe  <iain@sandoe.co.uk>

	* lib/target-supports.exp (check_effective_target_cet): Add the
	-fcf-protection flag to the build conditions.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271282 138bc75d-0d04-0410-961f-82ee72b054a4