Increase portability by using uintptr_t rather than size_t for pointers

The only integral types guaranteed by the C standard, if they exist, to
support having a pointer cast to them and back are (u)intptr_t. On most
architectures, size_t and uintptr_t are typedefs for the same underlying
type, so this code ends up working. However, on CHERI, and thus Arm's
experimental Morello prototype, C language pointers are implemented with
hardware capabilities, which are unforgeable pointers with bounds and
permissions. This means that, whilst size_t remains a plain 32/64-bit
integer size, (u)intotr_t is represented with a capability. Casting to
size_t and back to a pointer causes the capability metadata to be lost
and the resulting capability to be invalid, meaning it will trap when
dereferenced. Instead, use uintptr_t, and provide fallback definitions
for old versions of MSVC like for the other C99 integer types.

cmake/sysdep.h.in

@@ -33,6 +33,13 @@
     typedef unsigned __int32 uint32_t;
     typedef signed __int64 int64_t;
     typedef unsigned __int64 uint64_t;
+#   if defined(_WIN64)
+        typedef signed __int64 intptr_t;
+        typedef unsigned __int64 uintptr_t;
+#   else
+        typedef signed __int32 intptr_t;
+        typedef unsigned __int32 uintptr_t;
+#   endif
 #elif defined(_MSC_VER)  // && _MSC_VER >= 1600
 #   include <stdint.h>
 #else