go/gccgoexportdata: correctly handle archive files containing string tables

If the name of an archive member is longer than 16 bytes, an ELF archive
file contains a string table of file names under the name "//", and this
member has no mode.  Skip such members.

+ Test.

Change-Id: Ib10db1cc42816c9002433be6176240e490678560
Reviewed-on: https://go-review.googlesource.com/32973
Reviewed-by: Ian Lance Taylor <iant@golang.org>
4 files changed