debug-trap: use int3 on x86 instead of int $03

See Peter Cordes' comment on Stack Overflow:
https://stackoverflow.com/questions/173618/is-there-a-portable-equivalent-to-debugbreak-debugbreak/49079078#comment112451699_49079078

> For x86 (including x86-64) GAS syntax, it's better to write `int3` to
> make it explicit that you want the special case debug-break
> instruction, one byte `CC` not `CD 03`, for the rare cases where that
> matter (code size, and v8086 mode).
> (felixcloutier.com/x86/intn:into:int3:int1). With NASM they actually
> assemble differently, GAS optimizes both to int3.

debug-trap/debug-trap.h

@@ -33,7 +33,7 @@
 #  elif defined(__DMC__) && defined(_M_IX86)
      static inline void psnip_trap(void) { __asm int 3h; }
 #  elif defined(__i386__) || defined(__x86_64__)
-     static inline void psnip_trap(void) { __asm__ __volatile__("int $03"); }
+     static inline void psnip_trap(void) { __asm__ __volatile__("int3"); }
 #  elif defined(__thumb__)
      static inline void psnip_trap(void) { __asm__ __volatile__(".inst 0xde01"); }
 #  elif defined(__aarch64__)