* Added lowering hook for external weak global address. It inserts a load

  for Darwin.
* Added lowering hook for ISD::RET. It inserts CopyToRegs for the return
  value (or store / fld / copy to ST(0) for floating point value). This
  eliminate the need to write C++ code to handle RET with variable number
  of operands.

llvm-svn: 24888