[LLD][ELF] Silence warning when building with latest MSVC

This fixes:
```
[193/3517] Building CXX object
tools\lld\ELF\CMakeFiles\lldELF.dir\Arch\LoongArch.cpp.obj
C:\git\llvm-project\lld\ELF\Arch\LoongArch.cpp(683): warning C4334:
'<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit
shift intended?
```