MP_STATIC_ASSERT: Use _Static_assert.

.. or c++ static_assert where possible.

For the same reasons that C++ has trouble with "nonconstexpr"
static assertions, _Static_assert rejects such expression as well.
So, fall back to the old sizeof-array based implementation in that
case.

When _Static_assert can be used, the diagnostic quality is improved:
```
../py/objint.c: In function ‘mp_obj_int_make_new’:
../py/misc.h:67:32: error: static assertion failed: "37 == 42"
../py/objint.c:45:5: note: in expansion of macro ‘MP_STATIC_ASSERT’
```

As compared to a diagnostic about
```
../py/misc.h:71:50: error: size of unnamed array is negative
```

Testing on godbolt indicated that this actually works back to gcc
4.5, but it's easier to use GNUC >= 5 as the test; Hypothetical
users of 4.5, 4.6, or 4.7 will just get slightly worse diagnostics.

Related: micropython#18116

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

Author: jepler

py/misc.h

@@ -63,7 +63,14 @@ typedef unsigned int uint;
 #define MP_STRINGIFY(x) MP_STRINGIFY_HELPER(x)
 
 // Static assertion macro
+#if __cplusplus
+#define MP_STATIC_ASSERT(cond) static_assert((cond), #cond)
+#elif __GNUC__ >= 5 || __STDC_VERSION__ >= 201112L
+#define MP_STATIC_ASSERT(cond) _Static_assert((cond), #cond)
+#else
 #define MP_STATIC_ASSERT(cond) ((void)sizeof(char[1 - 2 * !(cond)]))
+#endif
+
 // In C++ things like comparing extern const pointers are not constant-expressions so cannot be used
 // in MP_STATIC_ASSERT. Note that not all possible compiler versions will reject this. Some gcc versions
 // do, others only with -Werror=vla, msvc always does.
@@ -72,7 +79,7 @@ typedef unsigned int uint;
 #if defined(_MSC_VER) || defined(__cplusplus)
 #define MP_STATIC_ASSERT_NONCONSTEXPR(cond) ((void)1)
 #else
-#define MP_STATIC_ASSERT_NONCONSTEXPR(cond) MP_STATIC_ASSERT(cond)
+#define MP_STATIC_ASSERT_NONCONSTEXPR(cond) ((void)sizeof(char[1 - 2 * !(cond)]))
 #endif
 
 // Round-up integer division