PR26936 testsuite fixes

Many targets fail this test due to -z noseparate-code not being
supported, or _start not being the proper entry symbol, or "as -g"
something other than "generate debug".

	PR 26936
	* testsuite/ld-elf/pr26936.d: Pass --gen-debug to gas rather than -g.
	Only run when -shared -z options are supported.
	* testsuite/ld-elf/pr26936b.s: Define more entry symbols.