[PowerPC][ELFv2ABI] Allocate parameter area on-demand to reduce stack frame size

This patch reduces the stack frame size by not allocating the parameter area if
it is not required. In the current implementation LowerFormalArguments_64SVR4
already handles the parameter area, but LowerCall_64SVR4 does not
(when calculating the stack frame size). What this patch does is make
LowerCall_64SVR4 consistent with LowerFormalArguments_64SVR4.

Committing on behalf of Hiroshi Inoue.

Differential Revision: https://reviews.llvm.org/D29881

llvm-svn: 296771