compiler: don't make map zero value constant
compiler: don't make map zero value constant

The map zero value is a common symbol, and it doesn't really make
sense to have a constant common symbol. Current GCC has started to
reject this case, probably as part of the fix for PR 83100.

Change-Id: I384ec8a58935d7021b94b11d6c76d3e37903dead
Reviewed-on: https://go-review.googlesource.com/80877
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend