fix another x86_64 ABI bug

The old code assumed that if an argument doesn't fit into the available
registers, none of the subsequent arguments do, either. But that's
wrong: passing 7 doubles, then a two-double struct, then another double
should generate code that passes the 9th argument in the 8th register
and the two-double struct on the stack. We now do so.

However, this patch does not yet fix the function calling code to do the
right thing in the same case.

x86_64-gen.c

@@ -1568,7 +1568,6 @@ void gfunc_prolog(CType *func_type)
                 addr = (addr + align - 1) & -align;
                 param_addr = addr;
                 addr += size;
-                sse_param_index += reg_count;
             }
             break;
             
@@ -1592,7 +1591,6 @@ void gfunc_prolog(CType *func_type)
                 addr = (addr + align - 1) & -align;
                 param_addr = addr;
                 addr += size;
-                reg_param_index += reg_count;
             }
             break;
         }