go/internal/gccgoimporter: additional V3 export data changes

This patch merges in support for reading the most recent
incarnation of V3 export data (initial inline function bodies),
from the importer portions of https://golang.org/cl/150061 and
https://golang.org/cl/150067.

Updates #28961.

Change-Id: I34e837acbf48b8fd1a4896a1a977d2241adfb28d
Reviewed-on: https://go-review.googlesource.com/c/151557


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>