[Bug go/63731] Fallback to netgo does not work

["boger at us dot ibm.com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63731

--- Comment #15 from boger at us dot ibm.com ---
I think what Ian is saying is that mechanism to rebuild packages in this way
doesn't work with gccgo (and probably never should?)

Now I'm finally understanding this.  Originally with gc the net package is
built with netgo off, but the netgo tag says to rebuild the GO standard library
with the netgo tag set on and then build the program.

Yohei's original fix used the code that was built with netgo off but allowed
the go resolver to be called if the call to getaddrinfo failed.  There are
probably a small set of errnos that could be checked to determine if the go
resolver should be called after getaddrinfo failed, but is that important?  I
would expect the errnos are consistent across platforms but don't know for
sure.

To me it seems like Yohei's fix is probably OK for gccgo with some additional
checks for errno if needed, but would change existing behavior for gc and
because of that should not change there?