PR26580, Size and alignment of commons vs as-needed shared lib

Two pieces to this puzzle:
1) Revert HJ's fix for PR13250 so that size and alignment isn't
   sticky, instead attack the real underlying problem that
   _bfd_generic_link_add_one_symbol does the wrong thing in making a
   common section in a shared library bfd.
2) Save and restore common u.c.p fields, which hold the section and
   alignment.

A better fix for (2) would be to throw away all of that horrible code
saving and restoring the hash table when loading as-needed library
symbols, and instead do a scan over as-needed library symbols before
adding anything.

bfd/
	PR 13250
	PR 26580
	* elflink.c (_bfd_elf_merge_symbol): Make "override" a bfd**.
	Return oldbfd in override when old common should override new
	common.
	(_bfd_elf_add_default_symbol): Adjust to suit.
	(elf_link_add_object_symbols): Likewise.  Pass "override" to
	_bfd_generic_link_add_one_symbol.  Save and restore common u.c.p
	field for --as-needed shared libraries.  Revert pr13250 changes.
ld/
	* testsuite/ld-elf/pr26580-a.s,
	* testsuite/ld-elf/pr26580-b.s,
	* testsuite/ld-elf/pr26580-1.sd,
	* testsuite/ld-elf/pr26580-2.sd: New tests
	* testsuite/ld-elf/comm-data.exp: Run new tests.
	* testsuite/ld-elf/pr26580-a.c,
	* testsuite/ld-elf/pr26580-b.c,
	* testsuite/ld-elf/pr26580-3.out,
	* testsuite/ld-elf/pr26580-4.out: New tests.
	* testsuite/ld-elf/shared.exp: Run new tests.

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