Allow tree-ssa.exp to be run by itself

Allow tree-ssa.exp to be run by itself

tree-ssa testcases sometimes check autovect effective target
but does not set it up.  On MIPS, those testcases fail with
some TCL error messages.  This fixes the issue by calling
check_vect_support_and_set_flags inside tree-ssa.exp.
There might be other .exp files which need to be done this
way too but I have not checked all of them.

Tested on x86_64-linux-gnu and a cross to mips64-octeon-linux-gnu.
Both full run of the testsuite and running tree-ssa.exp by itself.

testsuite/ChangeLog:
* tree-ssa.exp: Set DEFAULT_VECTCFLAGS and DEFAULT_VECTCFLAGS.
Call check_vect_support_and_set_flags also.