TarWriter: Only use 137 of the 155 prefix bytes.

This makes lld's --reproduce output more compatible with tar 1.13 and
before.  This is a very old version of tar, but it's the version in
both gnuwin and unxutils, and the cost for supporting them are very
low, so we might as well just do that.

https://bugs.chromium.org/p/chromium/issues/detail?id=1073524#c21
and onward has more details.

Differential Revision: https://reviews.llvm.org/D78945