*include/lightning.h, lib/lightning.c: Add the new jit_live code
to explicitly mark a register as live. It is required to avoid
assuming functions always return a value in the gpr and fpr return
register, and to avoid the need of some very specialized codes
that vary too much from backend to backend, to instruct the
optimization code the return register is live.
* lib/jit_arm.c, lib/jit_mips.c, lib/jit_ppc.c, lib/jit_print.c,
lib/jit_x86.c: Update for the new jit_live code.
* check/ret.ok, check/ret.tst: New files implementing a simple
test case that would previously fail at least in ix86/x86_64.
* check/Makefile.am: Update for new "ret" test case.
