pr20882 testcase

pr20882 testcase

-gdwarf-sections doesn't work on targets that lack support to emit
address size relative relocs.  The testcase as it was avoided
-gdwarf-sections doing anything by providing a non-empty .debug_line.
It's better to not use -gdwarf-sections.  Also, the testcase failed
to match the output for 16-bit address targets like avr.

	PR ld/20882
	* testsuite/ld-gc/pr20882.d: Don't pass -gdwarf-sections to gas.
	Allow for 16-bit address targets and match expected data fully.
	* testsuite/ld-gc/pr20882a.s: Delete .debug_line section.
	* testsuite/ld-gc/pr20882b.s: Likewise.
	* testsuite/ld-gc/pr20882c.s: Likewise.