mingw: Support for x86_64-w64-mingw32.

Until now, the assumption was made that POINTER and LONG are the same
size.  This is not so on x86_64-MinGW, which uses a 4-byte LONG and an
8-byte POINTER.  This patch introduces FIXNUM-SIZE, fixing x86_64-MinGW.

* module/system/base/target.scm (%native-word-size): Use sizeof long
instead of '*.  Fixes word size on x86_64-w64-mingw32
(%native-fixnum-size, %target-fixnum-size): New variable.
(triplet-pointer-size): Add case for mingw.
(target-fixnum-size): New procedure.
* libguile/bytevectors.c: Use SIZEOF_LONG > 4 instead of SIZEOF_VOID.
* libguile/vm-engine.c (INUM_MAX,INUM_MIN,INUM_STEP): Remove.
(BR_ARITHMETIC): Use scm_t_inum and SCM_I_INUM instead of
scm_t_signed_bits and SCM_UNPACK.
* module/system/vm/assembler.scm (<asm>)[ fixnum-size]: New field.
* module/system/vm/assembler.scm (make-assembler): Add #:fixnum-size
parameter.
(immediate-bits): Use fixnum-size for immediate size.