cmd/gc: support -installsuffix in the compiler and builder

cmd/gc: support -installsuffix in the compiler and builder

Add the -installsuffix flag to gc and {5,6,8}l, which overrides -race
for the suffix if both are supplied.
Pass this flag from the go tool for build and install.

R=rsc
CC=golang-dev
https://golang.org/cl/14246044