elf: Check symbol version without any symbols

VER_FLG_WEAK doesn't indicate that all symbol references of the symbol
version have STB_WEAK.  VER_FLG_WEAK indicates a weak symbol version
definition with no symbols associated with it.  It is used to verify
the existence of a particular implementation without any symbol references
to the weak symbol version.

	PR ld/24718
	* testsuite/ld-elf/pr24718-1.d: New file.
	* testsuite/ld-elf/pr24718-1.s: Likewise.
	* testsuite/ld-elf/pr24718-1.t: Likewise.