src/include/assert.h: add noreturn attribute to dead_code()

Clang does not recognize dead_code() as termination of execution. It
gives this message:

error: control reaches end of non-void function
      [-Werror,-Wreturn-type]

This change adds an __attribute__((noreturn)) to ensure that clang
recognises that this function will terminate execution.

This change is more general solution to the problem that was addressed
in the specific at https://review.coreboot.org/c/coreboot/+/32798

Signed-off-by: Alan Green <avg@google.com>
Change-Id: I5ba7189559aa01545d5bbe893bced400a3aaabbb
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32833
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/assert.h

@@ -56,7 +56,8 @@
  * bootmode.c:42: undefined reference to `dead_code_assertion_failed_at_line_42'
  */
 #define __dead_code(line) do { \
-	extern void dead_code_assertion_failed_at_line_##line(void); \
+	extern void dead_code_assertion_failed_at_line_##line(void) \
+		__attribute__((noreturn)); \
 	dead_code_assertion_failed_at_line_##line(); \
 } while (0)
 #define _dead_code(line) __dead_code(line)