[dsymutil] Replace PATH_MAX in SmallString with fixed value.

Apparently the Windows bots don't know this define, so just going with a
sensible default.

Failing builds:
  http://lab.llvm.org:8011/builders/lldb-x86-windows-msvc2015/builds/19179
  http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/19263

llvm-svn: 325762