fixup; hw: always serialize rdtsc reads

Since rdtsc() provides ordered timestamps now, we should reordering of
statements by the compiler too.

Issues genodelabs#5215, genodelabs#5430

repos/base-hw/src/include/hw/spec/x86_64/x86_64.h

@@ -121,6 +121,8 @@ struct Hw::Tsc
 			"rdtsc;"
 			"lfence;"
 			: "=a"(low), "=d"(high)
+			:
+			: "memory"
 		);
 		return (Genode::uint64_t)(high) << 32 | low;
 	}