cmd/6l: use .plt instead of .got on Solaris

Solaris requires all external procedures to be accessed through the
PLT. If 6l won't do it, /bin/ld will, so all the code written with .GOT
in mind won't work with the external linker.

This CL makes external linking work, opening the path to cgo support
on Solaris.

This CL breaks the Solaris build, this is fixed in subsequent CLs in
this series.

Change-Id: If370a79f49fdbe66d28b89fa463b4f3e91685f69
Reviewed-on: https://go-review.googlesource.com/8214


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>