sig-default x86_64 jit: add X86_ARGO_IMM variant

to put absolute values into the needed c-arg regs on amd64.
put c-arg in linear order, 0-n, default after the given args.

i386 fixes in next commit
sig-default i386 jit: add mov $regn, argn(%esp) to push immediate values onto the stack

also minor -m32 compilation fixes
docs: align jit comments

and fill in some missing pieces