Check unsupported .symver with common symbol

Check unsupported .symver with common symbol

The .symver directive on common symbol creates a new common symbol,
which shouldn't be allowed, similar to alias on common symbol:

$ cat y.S
	.comm	bar,8,8
	 .set bar1,bar
$ as -o y.o y.S
y.S: Assembler messages:
y.S:2: Error: `bar1' can't be equated to common symbol 'bar'
$

	PR gas/21661
	* config/obj-elf.c (obj_elf_symver): Don't allow .symver with
	common symbol.
	(elf_frob_symbol): Likewise.
	* testsuite/gas/elf/elf.exp: Run pr21661.
	* testsuite/gas/elf/pr21661.d: New file.
	* testsuite/gas/elf/pr21661.s: Likewise.