cmd/ld: fix incompatible type signatures on Plan 9

Changeset f483bfe81114 moved ELF generation to the architecture
independent code and in doing so added a Section* to the Sym
type and an Elf64_Shdr* to the Section type.

This caused the Plan 9 compilers to complain about incompatible
type signatures in the many files that reference the Sym type.

R=rsc, dave
CC=golang-dev
https://codereview.appspot.com/7057058