ld: Lookup section in output with the same name

ld: Lookup section in output with the same name

When there are more than one input sections with the same section name,
SECNAME, linker picks the first one to define __start_SECNAME and
__stop_SECNAME symbols.  When the first input section is removed by
comdat group, we need to check if there is still an output section
with section name SECNAME.

	PR ld/23591
	* ldlang.c (undef_start_stop): Lookup section in output with
	the same name.
	* testsuite/ld-elf/pr23591.d: New file.
	* testsuite/ld-elf/pr23591a.s: Likewise.
	* testsuite/ld-elf/pr23591b.s: Likewise.
	* testsuite/ld-elf/pr23591c.s: Likewise.