testsuite: Disable BTF and CTF testsuite on AIX.

testsuite: Disable BTF and CTF testsuite on AIX.

	CTF utilizes DWARF sections, but it is a distinct debugging
	format.  The CTF support in GCC is not implemented as a
	separate debugging format.  AIX supports DWARF but not CTF
	section.

	The GCC testsuite test for target support of a debugging format
	checks if GCC itself emits an error message, not if the
	debugging output compiles correctly.  Because CTF is not
	a distinct debugging format, GCC does not distinguish support
	for targets and does not have the ability to produce an error
	message.

	This patch skips the CTF and BTF debug directories, and
	explicitly reports that AIX doesn't support CTF.  Currently the
        dejagnu code to skip multiple debugging levels for CTF does
        not ensure that CTF is a supported debugging format. The patch
	also shifts the CTF options logic to within the test that CTF
	debug format is supported.

Bootstrapped on powerpc-ibm-aix7.2.3.0 and powerpc64le-linux.

gcc/testsuite/ChangeLog:

	* gcc.dg/debug/btf/btf.exp: Skip on AIX.
	* gcc.dg/debug/ctf/ctf.exp: Skip on AIX.
	* lib/gcc-dg.exp (gcc-dg-target-supports-debug-format): AIX
	doesn't support CTF.
	(gcc-dg-debug-runtest): Move CTF support within
	target support format test.