After removing absolute rpaths, cross compile builds (notably the nightly
builders) broke. This is because the RPATH was pointing at an empty directory
because only the rustc binary is copied over, not all of the target libraries.
This modifies the cross compile logic to fixup the rpath of the stage0
cross-compiled rustc to point to where it came from.
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.
