Don't make --enable-debug imply using the debug CRT in libffi

From Mozilla bug # 1014976 (https://bugzilla.mozilla.org/show_bug.cgi?id=1014976):

The current msvcc.sh setup requires linking against the debug MSVC CRT when -DFFI_DEBUG is set, which isn't strictly necessary (and makes life difficult when trying to use a different allocator). This decouples -DFFI_DEBUG from that via adding a new -DUSE_DEBUG_RTL flag so that the non-debug CRT can be used while still maintaining runtime error checks.