cmd/asm: add mode to collect symbol ABIs

This adds a -symabis flag that runs the assembler in a special mode
that outputs symbol definition and reference ABIs rather than
assembling the code. This uses a fast and somewhat lax parser because
the go_asm.h definitions may not be available.

For #27539.

Change-Id: I248ba0ebab7cc75dcb2a90e82a82eb445da7e88e
Reviewed-on: https://go-review.googlesource.com/c/147098


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