mk/prepare.mk: Don't try to install LLVMgold.so/libLTO.so for non-host targets.
librustc: PNaCl doesn't support split stacks.
librustc: Handle multiple-data types in debugging info generation, specifically in TypeMap.
librustc: Don't enable split stack support under PNaCl.
librustc: If we're not generating a stable pexe, don't run -strip-metadata or
the PNaCl verification passes.

If debugging infos are desired, stripping the module flags (as happens in
-strip-metadata) will later cause all LLVM tools to ignore dbg info in
bitcodes. Furthermore, we also must not run the verification passes because
module flags are invalid in PNaCl IR.
librustc: Adopt a hack employed by NaCl in LLVM. If targeting PNaCl, we pretend
we are ARM.