Avoid evaluating the sizeof in C preprocessor

The C preprocessor operates purely on text, substituting macros and
evaluating simple arithmetic expressions involving literal constants. It
does not understand types, so it cannot evaluate operators like sizeof
that require type information and compile-time type checking.

Change-Id: Id8b453db78571d67a8b7f95b240b71df212d7796

Author: jserv

CONTRIBUTING.md

@@ -766,15 +766,13 @@ typedef struct {
     uint16_t periodic  : 1; /* offset 6 bit 0      */
 } mytimer_t;
 
-/* Preprocessor check of timer register layout byte count. */
-#if (sizeof(mytimer_t) != 8)
-#error mytimer_t struct size incorrect (expected 8 bytes)
-#endif
+_Static_assert(sizeof(mytimer_t) == 8,
+               "mytimer_t struct size incorrect (expected 8 bytes)");
 ```
 
 To enhance portability, use standard-defined types (e.g., `uint16_t`, `uint32_t`) and avoid relying on compiler-specific behavior.
 Where precise control over memory layout is required, such as in embedded systems or when interfacing with hardware,
-always verify the structure size and layout using static assertions or preprocessor checks.
+always verify the structure size and layout using static assertions.
 
 #### Avoid extreme portability