package/go: Add HOST_GO_CGO_ENABLED

The go compiler's cgo support uses threads.  If BR2_TOOLCHAIN_HAS_THREADS is
set, build in cgo support for any go programs that may need it.  Note that
any target package needing cgo support must include
'depends on BR2_TOOLCHAIN_HAS_THREADS' in its config file.

Fixes build errors like these:

  error: #warning requested reentrant code

  http://autobuild.buildroot.net/results/42a8d07101d8d954511d1c884ecb66e8d861899e



Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>