asm: give error message for unadorned symbol reference

asm: give error message for unadorned symbol reference

On amd64, the program

TEXT    foo0(SB),7,$-8
    ADDQ R520, R1
    RET

used to trigger this error because R520 was being passed through to obj:

asm: doasm: notfound ft=23 tt=23 00000 (x.s:2)	ADDQ	0, 0 23 23

Now it gets this one, as it is indeed a parse error:

x.s:2: illegal addressing mode for symbol R520

This couldn't be fixed until #12632 had been fixed for arm64.

Fixes #12470.

Change-Id: I19830c4ae9337887b93f85d9a239e2b89dbb2219
Reviewed-on: https://go-review.googlesource.com/14691
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>