Log Message:
orc: update to 0.4.31.

0.4.31
======

  - Fix OrcTargetPowerPCFlags enum typedef to revert API change on
    macOS/iOS (Pablo Marcos Oltra)
  - Fixes for various PowerPC issues (Doug Nazar)
  - Enable flush-to-zero mode for float programs on ARM/neon (Doug Nazar)
  - Fix some opcodes to support x2/x4 processing on PowerPC (Doug Nazar)