Rpath: Use correct seperator & 'ORIGIN' variable

Fix two bugs in the CouchbaseRpath script which prevent
libjemalloc.2.dylib in install/lib from being picked up on OS X:

1) CMAKE_INSTALL_RPATH should be a CMake list, and hence semicolon
   seperated.
   (Aside: while the RPATH is encoded in ELF files as a
   semicolon-seperated string, for Mach-O RPATH is encoded as number
   of LC_RPATH commands, and hence CMAKE needs to see it as a list to
   pass the correct options to the linker).

2) On OSX '@executable_path' should be used instead of '$ORIGIN' when
   specifying the runtime location of a binary.

Change-Id: Ia25b2dee4f0f8fdf67cf2182a2889971a8ad3916
Reviewed-on: http://review.couchbase.org/43194
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>