[lld-macho] Work around odr-use of const non-inline static data member to fix...

[lld-macho] Work around odr-use of const non-inline static data member to fix -O0 build after D128298

```
ld.lld: error: undefined symbol: lld::macho::CodeSignatureSection::blockSize
>>> referenced by SyntheticSections.cpp:1253 (/home/maskray/llvm/lld/MachO/SyntheticSections.cpp:1253)
>>>               tools/lld/MachO/CMakeFiles/lldMachO.dir/SyntheticSections.cpp.o:(lld::macho::CodeSignatureSection::writeHashes(unsigned char*) const::$_7::operator()(unsigned long) const)
```