Provide option to avoid defining a custom version of uintptr_t.

There's a definition in stdint.h (provided by gcc) which will be more correct
if available.

Define CONFIG_USE_STDINT to use this feature, or USE_STDINT=1 on the 'make'
commmand.

This adjusts the settings for x86 and sandbox, with both have 64-bit options.

Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@google.com>
Rewritten to be an option, since stdint.h is often available only in glibc.
Changed to preserve a clear boundary between stdint and non-stdint
Signed-off-by: Simon Glass <sjg@chromium.org>