[Lightning] i386 sse support

[Paulo César Pereira de Andrade]

  Hi,

  In the latest code in the work branch of
http://github.com/pcpa/lightning/tree/work
it was enabled support for, and now it uses sse by
default on i386.

  It pass lightning's "make check" and the tests in
http://code.google.com/p/exl/source/browse/trunk/check/lightning
as well as the tests in my language (base of the above link),
but there, lightning is used currently almost only to glue
calls to C functions.

  But some work should be done, as currently, while the code
has different paths for x87, sse, i686, and some untested sse4.1
code, it still lacks actual runtime check, as well as support
for sse < 2, as the sse code assumes sse2 or newer.

  What do you think would be the proper way to detect the cpu
features, and add support for runtime reconfiguration?
Maybe there should be some kind of call, like
jit_get_cpu_features(), that one should call before generating
code, so that the macros jit_sse_p(), jit_i686_p(), etc,
could actually be runtime flags. Also, it may be desirable
to allow changing these flags, mainly for testing purposes.

Thanks,
Paulo