ld/elf: Ignore section symbols when matching linkonce with comdat

When deciding if a single member comdat group section in file FOO should
be discarded by a linkonce section in file BAR, we check if 2 sections
define the same set of local and global symbols.  When only one of the
files doesn't contain the unused section symbols in the symbol table,
such as object files generated by clang or GNU assembler with

commit d1bcae83
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Jan 7 06:42:00 2021 -0800

    ELF: Don't generate unused section symbols

the check will fail since one file has the extra unused section symbols.
We should ignore both undefined and section symbols in the symbol table
when making such a decision.

bfd/

	PR ld/27193
	* elflink.c (elf_create_symbuf): Also ignore section symbols.

ld/

	PR ld/27193
	* testsuite/ld-i386/i386.exp: Run PR ld/27193 test.
	* testsuite/ld-i386/pr27193.dd: New file.
	* testsuite/ld-i386/pr27193a.o.bz2: Likewise.
	* testsuite/ld-i386/pr27193b.s: Likewise.

(cherry picked from commit 994b2513)