Fix missing symbols in .rel/.rela.plt sections

Fix missing symbols in .rel/.rela.plt sections

Fix .rel.plt sections in the output to not only include .rel.plt
sections from the input but also the .rel.iplt sections and to define
the hidden symbols __rel_iplt_start and __rel_iplt_end around
.rel.iplt as otherwise we get undefined references to these when
linking statically to a multilib libc.a.  This fixes the static build
under i386.

Apply similar logic to rela.plt/.iplt and __rela_iplt/_plt_start/_end to
fix the static build under amd64.

Signed-off-by: Loïc Minier <lool@dooz.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>