[SystemZ][z/OS] Correct value of length/4 of params field in PPA1.

The Length/4 of Params field in the PPA1 ought to be the length of the parameters for the current function. Currently we are storing the length of the parameter area in the current function's stack frame, which represents the length of the params of the longest callee in the current function.

Differential revision: https://reviews.llvm.org/D119049

Reviewed By: uweigand