[SystemZ] Fix common-code users of stack size

On SystemZ we need to provide a register save area of 160 bytes to
any called function.  This size needs to be added when allocating
stack in the function prologue.  However, it was not accounted for
as part of MachineFrameInfo::getStackSize(); instead the back-end
used a private routine getAllocatedStackSize().

This is OK for code-gen purposes, but it breaks other users of
the getStackSize() routine, in particular it breaks the recently-
added -stack-size-section feature.

Fix this by updating the main stack size tracked by common code
(in emitPrologue) instead of using the private routine.

No change in code generation intended.

llvm-svn: 326610