Concerns have been raised about using absolute rpaths in #11746, and this is the
first step towards not relying on rpaths at all. The only current use case for
an absolute rpath is when a non-installed rust builds an executable that then
moves from is built location. The relative rpath back to libstd and absolute
rpath to the installation directory still remain (CFG_PREFIX).
Closes#11746
Rebasing of #12754
LLVM's tools are not contained in the local directory if --llvm-root is used by
the ./configure script. This fixes the installation path to be the root provided
by --llvm-root.
Because the build system treats Makefile.in and the .mk files slightly
differently (.in is copied, .mk are included), this makes the system
more uniform. Fewer build system changes will require a complete
reconfigure.
