cmd/asm: factor out line parsing from assembling

Currently cmd/asm's Parser.line both consumes a line of assembly from
the lexer and assembles it. This CL separates these two steps so that
the line parser can be reused for purposes other than generating a
Prog stream.

For #27539.
Updates #17544.

Change-Id: I452c9a2112fbcc1c94bf909efc0d1fcc71014812
Reviewed-on: https://go-review.googlesource.com/c/147097


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>