Workaround external linking issues encountered on Solaris 11.2+ due to
the go.o object file being created with a NULL STT_FILE symtab entry by
using a placeholder name.

Fixes #14957

Change-Id: I89c501b4c548469f3c878151947d35588057982b
Reviewed-on: https://go-review.googlesource.com/21636Reviewed-by: David Crawshaw <crawshaw@golang.org>