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 plugs a memory leak.

In particular remove the unnecessary 'sparc_is_valid_clobber()'.
The wrapper 'be_is_valid_clobber()' already handles 'memory' and 'cc'.

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.

It is like new_r_Proj(), but the mode is determined automatically from the out requirement of the predecessor.
This probably resolves some mode mismatches in untested code paths -- AMD64 backend and SPARC hard float, I'm looking at you!

Simplify the creation of instrinsic/runtime function entities a bit and avoid creating duplicate types.

We can easily identify the outputs producing a stack pointer by looking
if the stack pointer register is assigned. Unfortunately this also
needed a new mechanism to tell the verifier that some registers are not
in SSA form (all the time) which needs a few extra lines in each
backend.

Instead of having a generaic main loop in be_main with several callbacks
into the targets, split the code in be_main up into a set of convenience
functions and let the targets implement the main loop itself. This is
nicer because:

- The control flow into and out of the functions in the target becomes
  obvious as no external party is using callbacks anymore.
- Targets can insert custom code at any place between the generic
  passes.
- In the future we can parameterize the generic backend passes directly
  instead of stuffing info into generic isa_if structures.

This removes the last attributes from struct arch_env_t.

be: Move the info about the register classes and registers struct the arch_env_t into struct arch_isa_if_t.

Just use the selected isa interface directly.

Add callbacks on functions that really need them instead.

- alignment is set on each single IncSP now instead of a global arch_env
  setting
- fixed ia32 not requesting any alignment anymore (even on darwin where
  we need it)
- fixed amd64 creating IncSP nodes without flag_modifies_flags set

We now add missing keeps exactly before register allocation where we
actually need them (so that unused outputs which still need registers
assigned are not missed by the register allocation).

This removes functions from the public API which are pointless outside
localopt rules, and also avoids another struct in the public API.

- It was only available in the ia32 backend and broken there. As the
  feature is rarely used, I remove it for now instead of going through
  the trouble of fixing it.