[release-branch.go1.3] cmd/6g, cmd/8g: fix, test byte-sized magic multiply

««« CL 124950043 / 8e5ec6948793
cmd/6g, cmd/8g: fix, test byte-sized magic multiply

Credit to Rémy for finding and writing test case.

Fixes issue 8325.

LGTM=r
R=golang-codereviews, r
CC=dave, golang-codereviews, iant, remyoudompheng
https://codereview.appspot.com/124950043
»»»

TBR=rsc
CC=golang-codereviews
https://codereview.appspot.com/126000043