Use Mono prefix for libMonoPosixHelper

Use Mono prefix for libMonoPosixHelper

The libMonoPosixHelper is installed together with Mono, hence its path
in the configuration should be relative to the Mono prefix.

It was previously assumed to reside in a system path, so the the
dynamic linker would find it anyway.

This patch is based on the one included in bug #18555 (by Gaëtan
Lehmann <gaetan.lehmann@gmail.com>), but it also updates
runtime/Makefile.am to keep runtime/etc/mono/config consistent.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=18555