Rework of x86 function prolog and epilog

[Paulo César Pereira de Andrade]

  Hi,

  A rework of functions prolog and epilog has been done for x86.

  Most notable changes are:

o No fixed stack_framesize. This was from original lightning. Now it
  generates IR using a fixed stack_framesize but patches the offsets
  before code generation to reduce stack usage.
  It does so to avoid allow using an integer value to be both, a register
  number of an offset. If the offset is too large it knows it is a
  stack offset.
o In some cases it does not set the %rbp, only %rsp, as it saves and
  restores registers in prolog/epilog displaced from %rsp, and if there
  was no alloca used, %rbp is not really required.
  Unfortunately Lightning does not know about function attributes, so,
  if any non jit function is called, it still sets the %rbp value.
  make check pass all tests but catomic, if not setting %rbp. It crashes
  in pthread_create. Not certain about the reason, but for now it sets
  %rbp (creates a frame pointer) to avoid possible issues with unwind
  or related usage from a called function.
o The stack offset (CVT_OFFSET) to move from x87 to/from sse, or gpr to/from
  fpr is now dynamically allocated. It is very rarely used, and having
  it a fixed stack offset, even if not used, would mostly defeat this
  patch due to an implicit alloca in every function generated.

  Please let me know if you have any issues. It does not have issues in
all my usages, and passes "make check" in all x86 variants: Linux and
Windows x86_64, Linux x32, Linux and Windows 32 bit. But it is possible
that some usage has been broken. I can only think about code knowing
about Lightning internals and offsets of registers in the frame pointer;
if that is the case, using jit_frame() and jit_tramp() should provide
the previous behavior.

  The big advantage of this change is to make it cheaper to translate
some custom language function to a jit function, as the prolog/epilog
cost is drastically reduced now.

Thanks!
Paulo