GH-114847: Raise FileNotFoundError when getcwd() returns '(unreachable)' (#117481)

On Linux >= 2.6.36 with glibc < 2.27, `getcwd()` can return a relative
pathname starting with '(unreachable)'. We detect this and fail with
ENOENT, matching new glibc behaviour.

Co-authored-by: Petr Viktorin <encukou@gmail.com>