Successful `build_with_jit` produces broken binary on raspbian armhf

[nooga]

I'm not sure if this is pixie bug or pypy bug. I've built pixie on raspberry pi (raspbian jessie) according to the instructions for debian. The build succeeded and produced pixie-vm.

However, running pixie-vm produces the following error every time:

$ ./pixie-vm
[ARM/regalloc] gc_load_indexed_r not implemented
RPython traceback:
  File "rpython_jit_metainterp_compile.c", line 23729, in send_loop_to_backend
  File "rpython_jit_backend_arm_assembler.c", line 20717, in AssemblerARM_assemble_loop
  File "rpython_jit_backend_arm_assembler.c", line 23692, in AssemblerARM__assemble
  File "rpython_jit_backend_arm_assembler.c", line 26636, in AssemblerARM__walk_operations
  File "rpython_jit_backend_arm_regalloc.c", line 2152, in notimplemented
Fatal RPython error: NotImplementedError
Aborted

I tried to read pypy's sources to find out the cause but I can't see anything obvious.

[nooga]

I was able to build working jit on raspbian armhf by using more recent externals (rpython).
Quick chat with pypy maintainers confirmed that pixie master uses a version of rpython that didn't have gc_load_indexed_r implemented for arm.

I built a version from this commit: thomasmulvaney@90ea228

I think it might be a good idea to switch pixie master to use recent pypy.
I won't create a pull-request since this is @thomasmulvaney's fix.