libctf: don't run tests requiring deduplicating linker unless one is in use

libctf: don't run tests requiring deduplicating linker unless one is in use

Before now, we were relying on gcc -B to repoint GCC at our freshly-built
linker, and assuming we could use this to run tests that do linker dedup.

This is, alas, not reliable (building --with-ld=/an/absolute/path bypasses
-B searches for the linker), and we would like the libctf tests in
particular to not fail if the compiler uses a linker that cannot deduplicate
(simply skipping such tests).

Add a new testsuite function that tries to link one of the lookup tests'
testcases (which is known to produce conflicting types, i.e. a dict with
per-translation-unit children) and then does an objdump and greps it for
signs of the per-translation-unit children.  If there are none, the linker
probably just concatenated the sections: i.e., it is not a linker that
can deduplicate CTF.

libctf/
	PR libctf/33162
	* testsuite/config/default.exp (objdump): Introduce.
	(OBJDUMPFLAGS): Likewise.
	* testsuite/lib/ctf-lib.exp (check_ctf_linker_dedup): New.
	* testsuite/libctf-lookup/lookup.exp: Use it.
	* testsuite/libctf-regression/libctf-repeat-cu.exp: Likewise.
	* testsuite/libctf-regression/regression.exp: Likewise.