be: Rewrite stack frame handling
This is a bigger rewrite of stack frame handling in the backend: - Do the stack frame type layout late, after all the spill slots have been created. This way we can avoid (and remove) the brittle frame_alloc_area(). - Standardize the meaning of stack entity offsets. After stack frame type layout they are relative to the stack pointer position at the beginning of the function. It is all in one type now, no splitting into "arg", "between" and "stack_frame" type anymore. - Generalize the stack pointer simulation code to work with a single callback. Represent stack state as current offset and align_padding number. Now that callbacks can access and modify them both we do not need custom code in the sparc backend anymore. - Remove alignment specification on IncSP, only keep a ignore_align flag used for the last IncSP that has to remove all existing sp adjustments and may end up on a misaligned SP on ia32/amd64. - Align stack by default on ia32+amd64 backends.