PR26806, Suspected linker bug with LTO

PR26806, Suspected linker bug with LTO

This patch reverts most of git commit 1e3b96fd6cf, so IR symbols are
again not marked def_regular or ref_regular.  That should be enough to
stop IR symbols from becoming dynamic.  To mark as-needed shared
libraries referenced by IR symbols, use the referencing BFD rather
than the ref flags.

bfd/
	PR 15146
	PR 26314
	PR 26530
	PR 26806
	* elflink.c (elf_link_add_object_symbols): Don't set def/ref flags
	for plugin syms.  Do allow plugin syms to mark as-needed libs.
ld/
	PR 26806
	* testsuite/ld-plugin/lto-19.h,
	* testsuite/ld-plugin/lto-19a.c,
	* testsuite/ld-plugin/lto-19b.c,
	* testsuite/ld-plugin/lto-19c.c: New test.
	* testsuite/ld-plugin/pr26806.c,
	* testsuite/ld-plugin/pr26806.d: New test.
	* testsuite/ld-plugin/lto.exp: Run them.