Allow adding additional ldflags and other build flags
Allow adding additional ldflags and other build flags

For LinuxKit we build Go executables as static pie binaries, so
need to be able to add extra build and link flags.

Signed-off-by: Justin Cormack <af64ee19f04db9509cb02b0e470e11e5402ef9b3@docker.com>

vdemeester/containerd,lichao-mobanche/containerd,crosbymichael/containerd,AkihiroSuda/containerd,containerd/containerd,thaJeztah/containerd,estesp/containerd,miaoyq/containerd,dmcgowan/containerd,estesp/containerd,vdemeester/containerd,thaJeztah/containerd,hqhq/containerd,docker/containerd,dmcgowan/containerd,xiekeyang/containerd,mikebrow/containerd,kunalkushwaha/containerd,containerd/containerd,crosbymichael/containerd,rajasec/containerd,kunalkushwaha/containerd,mikebrow/containerd,chanezon/containerd,miaoyq/containerd