Correctly set relative rpath for OS X (#8266)

This includes both executables (by correcly setting the rpath to the
topDir) and libffi, and GHC itself, so that everything works with no
build tree.

Authored-by: Christiaan Baaj <christiaan.baaij@gmail.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>