Fix PPC32/64 ABI.

See http://llvm.org/bugs/show_bug.cgi?id=21087:
Static arrays were passed as e.g. [2 x i16]. The LLVM lowering uses a register for each entry,
extending it to 64bit. This is not the expected behaviour.
To work around this problem the PPC ABI now uses the byval-workaround from the Win64 ABI:
The struct/array is copied to some private memory and a pointer to this memory is passed to the function.
Structs/arrays of size 1/2/4/8 are cast to an integer and passed as integer.

This fixes all codegen crashes on Linux/PPC64le.
[cleanup] Removed unused struct stack from IRState.
Fixed getParentFunc(..., true) as used for nested context creation.

It seems like the original code contained two issues: First,
the stopOnStatic branch checked the passed symbol over and
over again instead of the current parent. Second, aggregate
declarations are not Declarations, they are ScopeDsymbols.

GitHub: Fixes #696.