This is simply a copy of std lib's go/internal/gcimporter/bimport.go
with updated header comment and build tag. No semantic changes.
This will fix part of the x/tools build break at tip. The other part
is to adjust bexport.go (next CL).
Change-Id: Ibc37fae7e0d0447fdea9e3a733aa38589735c59a
Reviewed-on: https://go-review.googlesource.com/21543
Reviewed-by: Alan Donovan <adonovan@google.com>