linux: Remove __stack_prot

The __stack_prot is used by Linux to make the stack executable if
a modules requires it.  It is also marked as RELRO, which requires
to change the segment permission to RW to update it.

Also, there is no need to keep track of the flags: either the stack
will have the default permission of the ABI or should be change to
PROT_READ | PROT_WRITE | PROT_EXEC.  The only additional flag,
PROT_GROWSDOWN or PROT_GROWSUP, is Linux only and can be deducted
from _STACK_GROWS_DOWN/_STACK_GROWS_UP.

Also, the check_consistency function was already removed some time
ago.

Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Florian Weimer <fweimer@redhat.com>