PR go/63731

	PR go/63731
libgo: Build and install libnetgo.a

libnetgo.a provides the net
package built with the netgo
tag enabled.  This provides the
netgo fallback solution for gccgo.
This lib must be explicitly linked
in using the -gccgoflags, so is
not included by default.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@221906 138bc75d-0d04-0410-961f-82ee72b054a4