golang: Fix ldflags when GO_PKG_LDFLAGS is set

golang: Fix ldflags when GO_PKG_LDFLAGS is set

go build/install supports multiple -ldflags arguments, but they are not
combined; for each package, the latest match on the command line is
used.[1]

Previously, the main executable would not be affected by the default
ldflags if GO_PKG_LDFLAGS or GO_PKG_LDFLAGS_X were set. (The default
ldflags instructs go to use the external linker.)

This fixes golang-package.mk so that the default ldflags take effect in
all cases.

[1]: https://golang.org/cmd/go/#hdr-Compile_packages_and_dependencies

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