cmd/asm: improve DATA size operand validation

Prior to this change, DATA instructions accepted
the values 1, 2, 4, and 8 as sizes.
The acceptable sizes were further restricted
to 4 and 8 for float constants.

This was both too restrictive and not restrictive enough:
string constants may reasonably have any length,
and address constants should really only accept pointer-length sizes.

Fixes #30269

Change-Id: I06e44ecdf5909eca7b19553861aec1fa39655c2b
Reviewed-on: https://go-review.googlesource.com/c/163747


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>