radeonsi: Cache LLVMTargetMachine object in si_screen

radeonsi: Cache LLVMTargetMachine object in si_screen

Rather than building a new one every compile.  This should reduce some
of the overhead of compiling shaders.

One consequence of this change is that we lose the MachineInstrs dumps
when dumping the shaders via R600_DEBUG.  The LLVM IR and assembly is
still dumped, and if you still want to see the MachineInstr dump, you
can run the dumped LLVM IR through llc.