Small fixes to Metasm x86_64 C Compiler

There were some bugs with incompatible size in the 64-bit C Compiler.

VERIFICATION Steps

```
c_code = <<-EOS
  void main(void){
    int foo = 5670;
    int bar = 8907;
    foo = foo * bar;
  }
EOS
 
#cpu = Metasm::Ia32.new
cpu = Metasm::X86_64.new
pe = Metasm::PE.compile_c(cpu ,c_code)
 ```

- [ ] Try to compile the above example before the fix, you'll get an Exception
- [ ] Run the above Compile example after the changes
- [ ] VERIFY it compiles successfully