Define intptr and uintptr in a more generic way

MSVC does not have the __PTRDIFF_TYPE__ macro defined, so I use the
standard [u]intptr_t types instead. Compilation tested on windows, linux
and mac.

BUG=
R=mark@chromium.org

Review URL: https://codereview.chromium.org/1571293003 .

Patch from Pavel Labath <labath@google.com>.

src/common/dwarf/types.h

@@ -45,11 +45,7 @@ typedef unsigned short     uint16;
 typedef unsigned int       uint32;
 typedef unsigned long long uint64;
 
-#ifdef __PTRDIFF_TYPE__
-typedef          __PTRDIFF_TYPE__ intptr;
-typedef unsigned __PTRDIFF_TYPE__ uintptr;
-#else
-#error "Can't find pointer-sized integral types."
-#endif
+typedef intptr_t           intptr;
+typedef uintptr_t          uintptr;
 
 #endif // _COMMON_DWARF_TYPES_H__