cmd/compile: avoid pointers in go.string.* symbols

cmd/compile: avoid pointers in go.string.* symbols

When creating binaries for dynamic linking, the linker moves
read-only data symbols that contain pointers into relro sections.
It is not setup for handling a go.string symbol moving to relro.

Instead of teaching it how (because go.string symbols with pointers
are unusual anyhow), put the data in a type.. section.

Fixes the android builder.

Change-Id: Ica4722d32241643c060923517b90276ff8ac6b07
Reviewed-on: https://go-review.googlesource.com/21110
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>