Revert part of b844a53a to fix ./perl -Ilib t/io/fs.t

The temps are created in t, but $ENV{PWD} points to root.

The change was made for Android support, but is now obsoleted
by newer code for Android, just below.
Negatives as UVs: sign-extension intentional, add cast.
NOTREACHED goes at/in the unreachable, not after it.