ld, testsuite, ctf: really test -gctf even if the compiler warns

ld, testsuite, ctf: really test -gctf even if the compiler warns

Commit a12c988767e5bd6b6a15dd6ca5e3b277f5627c64 endeavoured to
improve the CTF-availability test by having it try to create
CTF even if the compiler appears not to be working, checking
for the presence of likely-valid generated assembler instead.

Unfortunately this commit didn't remove the actual call to
check_compiler_available, so it didn't really improve anything.
Remove that call to induce proper testing on platforms in which
the compiler emits warnings.

ld/
	* testsuite/lib/ld-lib.exp (check_ctf_available): Don't
	require a working compiler before testing.