build: pass $GCFLAGS to compiler

build: pass $GCFLAGS to compiler

For example, if you are debugging an optimization
problem you can now run

        GCFLAGS=-N gotest

This is a convention for make, not for the general build,
so it may go away or be done differently in the eventual
'go' command.

The plan is that people will be able to test their code for
rune safety by doing GCFLAGS=-r.

R=golang-dev, bradfitz, lvd
CC=golang-dev
https://golang.org/cl/5294042