cmd/6l, cmd/internal/ld: handle R_PCREL to function in other shared library

An ELF linker handles a PC-relative reference to an STT_FUNC defined in a
shared library by building a PLT entry and referring to that, so do the
same in 6l.

Fixes #10690

Change-Id: I061a96fd4400d957e301d0ac86760ce256910e1d
Reviewed-on: https://go-review.googlesource.com/9711


Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>