linux x86 and amd64 memory protection key syscalls.

linux x86 and amd64 memory protection key syscalls.

This implements minimal support for the pkey_alloc, pkey_free and
pkey_mprotect syscalls. pkey_alloc will simply indicate that pkeys
are not supported. pkey_free always fails. pkey_mprotect works just
like mprotect if the special pkey -1 is provided.

https://bugs.kde.org/show_bug.cgi?id=408091