Use (u)intptr_t in khrplatform.h if available (#137)

While most architectures allow casting a pointer to `long`/`long long`
and back, this does not work on CHERI-enabled architectures such as
e.g. Arm Morello, where pointers are 128-bits but `long long` is still 64.
We use (u)inptr_t when possible to handle more platforms. Ideally, we
could just use (u)intptr_t everywhere, but this could result in ABI
breakage if khronos_uintptr_t is changed from unsigned long to
unsigned long long or similar (this results in different C++ name
mangling). To avoid changes for existing platforms, we restrict usage
of intptr_t to platforms where the size of a pointer is larger than the
size of long.

api/KHR/khrplatform.h

@@ -153,6 +153,20 @@ typedef int64_t                 khronos_int64_t;
 typedef uint64_t                khronos_uint64_t;
 #define KHRONOS_SUPPORT_INT64   1
 #define KHRONOS_SUPPORT_FLOAT   1
+/*
+ * To support platform where unsigned long cannot be used interchangeably with
+ * inptr_t (e.g. CHERI-extended ISAs), we can use the stdint.h intptr_t.
+ * Ideally, we could just use (u)intptr_t everywhere, but this could result in
+ * ABI breakage if khronos_uintptr_t is changed from unsigned long to
+ * unsigned long long or similar (this results in different C++ name mangling).
+ * To avoid changes for existing platforms, we restrict usage of intptr_t to
+ * platforms where the size of a pointer is larger than the size of long.
+ */
+#if defined(__SIZEOF_LONG__) && defined(__SIZEOF_POINTER__)
+#if __SIZEOF_POINTER__ > __SIZEOF_LONG__
+#define KHRONOS_USE_INTPTR_T
+#endif
+#endif
 
 #elif defined(__VMS ) || defined(__sgi)
 
@@ -235,14 +249,21 @@ typedef unsigned short int     khronos_uint16_t;
  * pointers are 64 bits, but 'long' is still 32 bits. Win64 appears
  * to be the only LLP64 architecture in current use.
  */
-#ifdef _WIN64
+#ifdef KHRONOS_USE_INTPTR_T
+typedef intptr_t               khronos_intptr_t;
+typedef uintptr_t              khronos_uintptr_t;
+#elif defined(_WIN64)
 typedef signed   long long int khronos_intptr_t;
 typedef unsigned long long int khronos_uintptr_t;
-typedef signed   long long int khronos_ssize_t;
-typedef unsigned long long int khronos_usize_t;
 #else
 typedef signed   long  int     khronos_intptr_t;
 typedef unsigned long  int     khronos_uintptr_t;
+#endif
+
+#if defined(_WIN64)
+typedef signed   long long int khronos_ssize_t;
+typedef unsigned long long int khronos_usize_t;
+#else
 typedef signed   long  int     khronos_ssize_t;
 typedef unsigned long  int     khronos_usize_t;
 #endif