osdep.h: Define macros for the benefit of C++ before C++11

osdep.h: Define macros for the benefit of C++ before C++11

For C++ before C++11, <stdint.h> requires definition of the macros
__STDC_CONSTANT_MACROS, __STDC_LIMIT_MACROS and __STDC_FORMAT_MACROS
in order to enable definition of various macros by the header file.
Define these in osdep.h, so that we get the right header file
definitions whether osdep.h is being used by plain C, C++11 or
older C++.

In particular libvixl's header files depend on this and won't
compile if osdep.h is included before them otherwise.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>