[lld-macho] Use uint64_t instead of size_t to fix 32 bit test failures

Our bot has been failing https://lab.llvm.org/buildbot/#/builders/178/builds/3967:
Assertion `isecEnd - isecVA <= forwardBranchRange && "should only finalize sections in jump range"' failed.

I think this is due to the use of size_t, which is 32 bit on 32 bit,
for a value used in some 64 bit address calculations. Which was added in
https://reviews.llvm.org/D144029.

Switching to uint64_t fixes the issues.