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

arm, be, ia32, sparc: Do not go on with peephole optimization of IncSP, when be_peephole_IncSP_IncSP() succeeded.

The current IncSP was removed in this case and we now would operate on its predecessor IncSP.
This is wrong, e.g. when asking for currently free registers.
This corrects backend/incsp_pop.c, which was broken on ia32.
It was wrongly assumed that a certain register was free and could be used to turn an IncSP into a Pop.

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!

- Rename other_same field to should_be_same.
- Remove arch_register_req_type_other_same, it's enough to see if the
  should_be_same field is not zero.

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.

In case of endless loops, there is no user of Start, so the missing keeps walker keeps Start's results.
Then the be_keep_if_unused() workaround does not trigger, because after rewiring the initial SP's users (here the Keep) to the IncSP it has one user.
But the IncSP is placed after the Keep in the schedule.
This either makes the verifier unhappy (arm) or gets erroneously removed by the dead nodes walker (sparc).
This solution is not entierly correct, but is consistent with what the ia32 backend does.
The correct solution would be a memory output for IncSP.

Checking the register implicitly covers checking for a data mode.

Add callbacks on functions that really need them instead.

* No \n at end; panic() adds one
* No punctuation mark at end
* Lowercase first letter

* It was retrieved from the arch_env before.
* Also rename be_abi_fix_stack_nodes() to be_fix_stack_nodes().

All other structs holding a register class call it cls, too.

- We now create a spill/reload/phi-web and take the largest necessary
  reload size to determine the size of the entities and the spills.
- Some cleanups

This mainly adds comments explaining the situation and avoids an extra
graph walk.

This works around a problem with modeling control flow for calls to
setjmp and similar functions.

It has been empty for nearly all systems. People who used to put stuff
in config.h can still create a config.h on their own and inject
-include (gcc) or /FI (msvc) into their CPPFLAGS.