cmd/compile: accept and parse symabis

This doesn't yet do anything with this information.

For #27539.

Change-Id: Ia12c905812aa1ed425eedd6ab2f55ec75d81c0ce
Reviewed-on: https://go-review.googlesource.com/c/147099


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>