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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.

They previously used be_stack_layout but that structure will be remove
soon.

This allows us to properly implement the OS X calling conventions.

This fixes backend/long_double_arith.c.

This plugs a memory leak.

Commit d33e0668 accidentally disabled
varargs support unless long double support is enabled.

Also disable it by default for now until x87 support is more mature.

We can see that we need a frame entity if the the immediate has a
X86_IMM_FRAMEOFFSET relocation but no entity assigned yet.

be: Pass the input requirements directly to the constructors of nodes, which have a variable number of arguments.

This adds two builtin kinds, ir_bk_va_start and ir_bk_va_arg. va_arg Builtins
are lowered in the lower_builtins pass with a function provided by the backend,
whereby most architectures can use be_default_lower_va_args. va_start Builtins
are lowered in the backend.

Note: This commit breaks all backends except for IA32. SPARC and AMD64 are
implemented in later commits. The ARM backend previously miscompiled variadic
functions, and will now throw a proper error.

All backends have similar empty Start nodes, which just provide registers.
So having one generic backend Start node to share some infrastructure is sensible.

It pops a value from the stack, so this memory access should be represented in the graph.