Fix the signal code on 32-bit breakpoints on arm64

When debugging 32-bit programs a debugger may insert a instruction that
will raise the undefined instruction trap. The kernel handles these
by raising a SIGTRAP, however the code was incorrect.

Fix this by using the expected TRAP_BRKPT signal code.

Sponsored by:	The FreeBSD Foundation

sys/arm64/arm64/undefined.c

@@ -105,7 +105,7 @@ gdb_trapper(vm_offset_t va, uint32_t insn, struct trapframe *frame,
 
 			ksiginfo_init_trap(&ksi);
 			ksi.ksi_signo = SIGTRAP;
-			ksi.ksi_code = TRAP_TRACE;
+			ksi.ksi_code = TRAP_BRKPT;
 			ksi.ksi_addr = (void *)va;
 			trapsignal(td, &ksi);
 			return 1;