cmd/internal/ld: put all type decoding in decodesym.go

Move the one instance of type structure decoding in the linker that
doesn't live decodesym.go in to decodesym.go.

Change-Id: Ic6a23500deb72f0e9c8227ab611511e9781fac70
Reviewed-on: https://go-review.googlesource.com/9690


Reviewed-by: Russ Cox <rsc@golang.org>