cmd/asm: make asm -S flag consistent with compile -S flag

Change things so that the -S command line option for the assembler
works the same as -S in the compiler, e.g. you can use -S=2 to
get additional detail.

Change-Id: I7bdfba39a98e67c7ae4b93019e171b188bb99a2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/255717


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>