MC: Support COFF string tables larger than 10MB

Offsets past the range of single-slash encoding are encoded as base64,
padded to 6 characters, and prefixed with two slashes. This encoding is
undocumented but used by MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201940 91177308-0d34-0410-b5e6-96231b3b80d8