work around breakage in sh/fdpic __unmapself function

work around breakage in sh/fdpic __unmapself function

the C implementation of __unmapself used for potentially-nommu sh
assumed CRTJMP takes a function descriptor rather than a code address;
however, the actual dynamic linker needs a code address, and so commit
7a9669e977e5f750cf72ccbd2614f8b72ce02c4c changed the definition of the
macro in reloc.h. this commit puts the old macro back in a place where
it only affects __unmapself.

this is an ugly workaround and should be cleaned up at some point, but
at least it's well isolated.