cmd/asm, go/build: invoke cmd/asm only once per package

cmd/asm, go/build: invoke cmd/asm only once per package

Prior to this CL, cmd/go invoked cmd/asm once
for every assembly file.
The exec and cmd/asm startup overhead dwarfed
the actual time spent assembling.
This CL adds support to cmd/asm to process
multiple input files and uses it in cmd/go.

This cuts 10% off the wall time for 'go build -a math'.

Fixes #15680

Change-Id: I12d2ee2c817207954961dc8f37b8f2b09f835550
Reviewed-on: https://go-review.googlesource.com/27636
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>