This adds several variables for Go package Makefiles:
* GO_PKG_GCFLAGS - go tool compile arguments
* GO_PKG_LDFLAGS - go tool link arguments
* GO_PKG_LDFLAGS_X - go tool link -X definitions

Settings these will add the corresponding flags to the go install
command line. (Other command line arguments can still be added by
passing them as the first argument to GoPackage/Build/Compile.)

This also adds Go's runtime environment variables (GOGC, GOMAXPROCS,
GOTRACEBACK) to the unexport list.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>