Don't check shared/export_dynamic/ref_dynamic for type mismatch

Don't check shared/export_dynamic/ref_dynamic for type mismatch

There is nothing linker can do when a type mismatched default definition
are made dynamic by info->shared, info->export_dynamic or h->ref_dynamic.
But we do want to avoid exporting it when building PIE.  Let's remove
those checks.

bfd/

	PR ld/2404
	* elflink.c (_bfd_elf_merge_symbol): Don't check info->shared,
	info->export_dynamic, nor !h->ref_dynamic for type mismatch when
	adding the default version.

ld/testsuite/

	PR ld/2404
	* ld-elf/shared.exp: Add a PIE test for PR ld/2404.