FFI support for 64bit architectures.

FFI support for 64bit architectures.

* src/ffi/generator.scm: Declare long callback IDs, large enough to
avoid truncation warnings when the toolkit type is also large.  Use
%ld instead of %d for sizeof and member offsets, and cast them to
long.  On x86_64, these have types ulong and long respectively.

* src/microcode/: pruxffi.c, pruxffi.h: (callback_run_kernel)
(callback_run_handler): Declare long callback_id params.  Use %ld.

* src/runtime/ffi.scm (radix): Parameterize alien operations to
support 64bit or 32bit addresses.