Handle relocatable libMonoPosixHelper.so when --libdir= isn't lib/

Handle relocatable libMonoPosixHelper.so when --libdir= isn't lib/

Right now, we use a special token in mono/config which replaces $mono_libdir with the runtime-detected assemblies lib dir - e.g. /usr/lib

Unfortunately, this does not handle cases where a --libdir= value is passed to configure, such as on Red Hat where they use --libdir=/usr/lib64 on AMD64

We already have a variable from configure - reloc_libdir - so introduce some helpers to use it in the $mono_libdir replacement.

This assumes the value of --libdir is a subdirectory of --prefix, but I doubt we have ever worked at all when that is not the case.

Fixes: #41953