cmd/go: improvements

cmd/go: improvements

Do not treat $GOROOT/src/pkg, $GOROOT/src/cmd,
$GOPATH/src as package directories (only subdirectories
of those can be package directories).  Fixes issue 2602.

Accept additional compiler and linker arguments during
cgo from $CGO_CFLAGS and $CGO_LDFLAGS, as the
Makefiles used to do.

Show failed pkg-config output.  Fixes issue 2785.

Use different (perhaps better) git commands.  Fixes issue 2109.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5605045