ld: Skip libdep plugin if not all plugin hooks are available

Skip plugin if not all required plugin hooks are available.

2020-12-16  Howard Chu <hyc@symas.com>
	    H.J. Lu  <hongjiu.lu@intel.com>

	PR ld/27081
	* libdep_plugin.c (onload): Skip if not all required plugin hooks
	are available.
	* testsuite/config/default.exp (dep_plug_opt): New.
	* testsuite/ld-elf/elf.exp: Pass $dep_plug_opt to nm.
	* testsuite/ld-elf/pr26391.fd: New file.