DEBUG: crash using an invalid opcode on aarch64 instead of an invalid access

On aarch64 there's also a guaranted invalid instruction, called UDF, and
which even supports an optional 16-bit immediate operand:

   https://developer.arm.com/documentation/ddi0596/2021-12/Base-Instructions/UDF--Permanently-Undefined-?lang=en

It's conveniently encoded as 4 zeroes (when the operand is zero). It's
unclear when support for it was added into GAS, if at all; even a
not-so-old 2.27 doesn't know about it. Let's byte-encode it.

Tested on an A72 and works as expected.

include/haproxy/bug.h

@@ -54,6 +54,13 @@
 		my_unreachable();					\
 	} while (0)
 
+#elif defined(__aarch64__)
+#define ha_crash_now() do {						\
+		/* udf#imm16: 4 bytes (), raises illegal instruction */	\
+		__asm__ volatile(".byte 0x00,0x00,0x00,0x00\n");	\
+		my_unreachable();					\
+	} while (0)
+
 #else // not x86
 
 /* generic implementation, causes a segfault */