x86-64-asm: Support high registers %r8 - %r15

This requires correctly handling the REX prefix.
As bonus we now also support the four 8bit registers
spl,bpl,sil,dil, which are decoded as ah,ch,dh,bh in non-long-mode
(and require a REX prefix as well).

i386-tok.h

@@ -93,6 +93,14 @@
  DEF_ASM(st)
  DEF_ASM(rip)
 
+#ifdef TCC_TARGET_X86_64
+ /* The four low parts of sp/bp/si/di that exist only on
+    x86-64 (encoding aliased to ah,ch,dh,dh when not using REX). */
+ DEF_ASM(spl)
+ DEF_ASM(bpl)
+ DEF_ASM(sil)
+ DEF_ASM(dil)
+#endif
  /* generic two operands */
  DEF_BWLX(mov)