cmd/link: add .go.buildinfo in XCOFF symbol table

cmd/link: add .go.buildinfo in XCOFF symbol table

.go.buildinfo must be added to the symbol table on AIX. Otherwise, ld
won't be able to handle its relocations.

This patch also make ".data" the default section for all symbols inside
the data segment.

Change-Id: I83ac2bf1050e0ef6ef9c96ff793efd4ddc8e98d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/174298
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>