package/pkg-golang.mk: add osusergo and netgo tags in static builds

CGO (the Go language C bindings) normally links dynamically with the C
libraries. It seems that passing -extldflags=-static is not sufficient
to make sure they link statically. As a result, a build using CGO will
fail with

runtime.gcdata: missing Go type information for global symbol .dynsym: size 72

There are two modules in the standard Go library that can use CGO:
os/user and net. Both of these have pure Go versions, however, which can
be selected with the osusergo and netgo tags. Since these two are
standard library modules that get used in almost all Go programs, pass
those flags in the common infrastructure.

It's not entirely clear if this is always the case or only depending on
the specific way CGO is used. In the former case, we could instead
disable CGO globally if static libs are enabled. However, it turns out
that this doesn't solve the problem completely anyway (delve still fails
to link, with different errors). Also, it is possible that other cgo
modules do still work. Therefore, just stick to adding the osusergo and
netgo tags.

Cf. Statically compiling Go programs [0]

Fixes:
 - crucible:
   http://autobuild.buildroot.net/results/c2f/c2f29f55b461899e8e15c0494578304e6f74bf51/
 - containerd:
   http://autobuild.buildroot.net/results/699/69910fc6b9f474a7eb0ca7ffa27ac5512cc1bb37/
 - delve:
   http://autobuild.buildroot.net/results/552/55225d5da17e85271943f7adce6f07d014d9717d/
 - probably others...

[0] https://www.arp242.net/static-go.html

Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit c6bb759e2c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-golang.mk

@@ -102,6 +102,7 @@ ifeq ($(4),target)
 
 ifeq ($(BR2_STATIC_LIBS),y)
 $(2)_LDFLAGS += -extldflags '-static'
+$(2)_TAGS += osusergo netgo
 endif
 
 # Build package for target