64-bit support for Pelles C:
* New nmake file w3i6pc.nmk.
* New stack scanner ssw3i6pc.c supplies missing jump buffer definition.
* Some platform tests change from defined(MPS_PF_W3I6MV) to defined(MPS_OS_w3) && defined(MPS_ARCH_I6).
* Make reasonable changes to the source code to avoid warnings from Pelles C:
** Ensure that printf formats are consistent with arguments by using PRIuLONGEST and casting to ulongest_t.
** Use size_t for loop variables that index into arrays.
** Suppress "Consider changing type to 'size_t' for loop variable" warnings for the MPS core.