rs6000, testsuite: Correct the broken_cplxf_arg test

rs6000, testsuite: Correct the broken_cplxf_arg test

check_effective_target_broken_cplxf_arg is buggy.  It actually tests
if passing complex float works, not if it fails.  Also, it only runs
the test for target powerpc64-linux, but we are biarch, so it should
be powerpc*-linux.

This also changes the early-out conditions to be separate, because
the big combined condition was hard to follow.

This fixes the libstdc++ tests:

Running target unix/-m64
XPASS: 26_numerics/complex/13450.cc execution test
XPASS: 26_numerics/complex/pow.cc execution test
XPASS: 26_numerics/complex/value_operations/1.cc execution test

which were the last failing libstdc++ tests on BE.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_broken_cplxf_arg):
	Fix test.  Make early-out condition return early.  Correct comments.

From-SVN: r246072