Provide <bits/unistd_ext.h> as a sysdeps header exclusively

Non-sysdeps headers cannot be overriden by sysdeps headers across the
entire build, so it is necessary to turn such extension headers into
sysdeps headers themselves.  The approach here follows the existing
<bits/shm.h> header (although it uses sysdeps/gnu instead of
sysdeps/generic).

Fixes commit 1d0fc213 ("Linux: Add
gettid system call wrapper [BZ #6399]") and commit
8f89ab21 ("posix: Fix missing wrapper
header for <bits/unistd_ext.h>").