SDL_assert.h: add inline asm (int $3) as SDL_TriggerBreakpoint for Watcom/x86

(also disable SIGTRAP case to !watcom, because watcom doesn't have SIGTRAP.)

Partially fixes Bugzilla #3758.

include/SDL_assert.h

@@ -53,7 +53,9 @@ assert can have unique static variables associated with it.
     #define SDL_TriggerBreakpoint() __debugbreak()
 #elif ( (!defined(__NACL__)) && ((defined(__GNUC__) || defined(__clang__)) && (defined(__i386__) || defined(__x86_64__))) )
     #define SDL_TriggerBreakpoint() __asm__ __volatile__ ( "int $3\n\t" )
-#elif defined(HAVE_SIGNAL_H)
+#elif defined(__386__) && defined(__WATCOMC__)
+    #define SDL_TriggerBreakpoint() { _asm { int 0x03 } }
+#elif defined(HAVE_SIGNAL_H) && !defined(__WATCOMC__)
     #include <signal.h>
     #define SDL_TriggerBreakpoint() raise(SIGTRAP)
 #else