cmd/release: set CGO_{C,LD}FLAGS if GOARM is set

So it really builds an ARMv6 binary when GOARM=6.

Tried "release -rev go1.8rc2 -tools release-branch.go1.8 -version go1.8rc2 -target linux-armv6l -watch"
and the generated binary works on my Raspberry Pi 1.

Fixes golang/go#18748.

Change-Id: I11113f98703049dc9990fe8c8abf505dcfd980f2
Reviewed-on: https://go-review.googlesource.com/35501
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/release/release.go

@@ -348,6 +348,8 @@ func (b *Build) make() error {
 
 	if b.Goarm > 0 {
 		env = append(env, fmt.Sprintf("GOARM=%d", b.Goarm))
+		env = append(env, fmt.Sprintf("CGO_CFLAGS=-march=armv%d", b.Goarm))
+		env = append(env, fmt.Sprintf("CGO_LDFLAGS=-march=armv%d", b.Goarm))
 	}
 
 	// Execute build