[hurd] Fix unconditional use of PATH_MAX

Patch by Samuel Thibault

The GNU/Hurd system does not define an arbitrary PATH_MAX limitation, the POSIX 2001 realpath
extension can be used instead, and the size of symlinks can be determined.

Reviewed as https://reviews.llvm.org/D54677

llvm-svn: 351414
GitOrigin-RevId: 0e997efd53e839c6f34d470ecf89bd1c734ea94b