[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)
```

Author: MaskRay

lld/MachO/SyntheticSections.cpp

@@ -1250,7 +1250,8 @@ void CodeSignatureSection::writeHashes(uint8_t *buf) const {
   uint8_t *hashes = buf + fileOff + allHeadersSize;
   parallelFor(0, getBlockCount(), [&](size_t i) {
     sha256(buf + i * blockSize,
-           std::min(static_cast<size_t>(fileOff - i * blockSize), blockSize),
+           std::min(static_cast<size_t>(fileOff - i * blockSize),
+                    static_cast<size_t>(blockSize)),
            hashes + i * hashSize);
   });
 #if defined(__APPLE__)