[LLD][MachO] Fix warning when building with latest MSVC
This fixes:
```
[3902/4335] Building CXX object tools\lld\MachO\CMakeFiles\lldMachO.dir\Arch\X86_64.cpp.obj
C:\git\llvm-project\lld\MachO\Arch\X86_64.cpp(107): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
```
