go/internal/gccgoimporter: use a slice instead of a map for type map (optimization)

go/internal/gccgoimporter: use a slice instead of a map for type map (optimization)

ggcgo's export format numbers types consecutively, starting at 1.
This makes it trivially possible to use a slice (list) instead of
map for the internal types map.

Change-Id: Ib7814d7fabffac0ad2b56f04a5dad7d6d4c4dd0e
Reviewed-on: https://go-review.googlesource.com/137935
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>