Although sizeof(void *) == sizeof(size_t) for all architectures that are
currently supported by libjpeg-turbo, such is not guaranteed by the C
standard.  Specifically, CHERI-enabled architectures (e.g. CHERI-RISC-V
or Arm's Morello) use capability pointers that are twice the size of
size_t (128 bits for Morello and RV64), so casting to size_t strips the
upper bits of the pointer (including the validity bit) and makes it
non-deferenceable, as indicated by the following compiler warning:

  warning: cast from provenance-free integer type to pointer type will
  give pointer that can not be dereferenced
  [-Werror,-Wcheri-capability-misuse]
    cvalue = values = (JCOEF *)PAD((size_t)values_unaligned, 16);

Ignoring this warning results in a run-time crash.  Casting pointers to
uintptr_t, if it is available, avoids this problem, since uintptr_t is
defined as an unsigned integer type that can hold a pointer value.

Since C89 compatibility is still necessary in libjpeg-turbo, this commit
introduces a new typedef for pointer-to-integer casts that uses a
GNU-specific extension available in GCC 4.6+ and Clang 3.0+ and falls
back to using size_t if the extension is unavailable.  The only other
options would require C99 or Clang-specific builtins.

Closes #538