gdb/testsuite: Introduce skip_ctf_tests guard function

gdb/testsuite: Introduce skip_ctf_tests guard function

Most versions of GCC in the wild don't support CTF debug format right
now, so, rather than attempting to compile the tests and failing each
time, this patch introduces a guard function to check if the compiler
supports CTF.  If we don't have CTF support then the CTF tests are
skipped.

This patch only updates 3 of the 4 CTF tests, the fourth will be
handled in the next patch.

gdb/testsuite/ChangeLog:

	* gdb.base/ctf-constvars.exp: Skip test if CTF is not supported in
	the compiler.  Clean up header comment a little.
	* gdb.base/ctf-ptype.exp: Likewise.
	* gdb.base/ctf-whatis.exp: Likewise.
	* lib/gdb.exp (skip_ctf_tests): New proc.

Change-Id: I505c11169a9bc9871a31fc0c61e119f92f32cc63