x86_64: Relax ltr operand constraint

Any 16 bit general puprose register can be a ltr operand, no need
to force AX.

src/system/kernel/arch/x86/64/descriptors.cpp

@@ -34,7 +34,7 @@ static inline void
 load_tss(int cpu)
 {
 	uint16 segment = (TSS_SEGMENT(cpu) << 3) | DPL_KERNEL;
-	asm volatile("ltr %w0" : : "a" (segment));
+	asm volatile("ltr %w0" : : "r" (segment));
 }