x86-64-asm: Clean up 64bit immediate support

Fix it to actually be able to parse 64bit immediates (enlarge
operand value type).  Then, generally there's no need for accepting
IM64 anywhere, except in the 0xba+r mov opcodes, so OP_IM is
unnecessary, as is OPT_IMNO64.  Improve the generated code a bit
by preferring the 0xc7 opcode for im32->reg64, instead of the
im64->reg64 form (which we therefore hardcode).

tests/asmtest.S

@@ -46,6 +46,16 @@ movl %eax, 0x100(%ebx,%edx,2)
 movl 0x100(%ebx,%edx,2), %edx
 movw %ax, 0x100(%ebx,%edx,2)
 
+movw $0x1122,%si
+movl $0x112233,%edx
+#ifdef __x86_64__
+mov $0x11223344,%rbx
+movq $0x11223344,%rbx
+mov $0x1122334455,%rbx
+movq $0x1122334455,%rbx
+movl $0x11334455,(%rbx)
+#endif
+
 mov %eax, 0x12(,%edx,2)
         
 #ifdef __i386__