selftests/mm: Add new testcases for pkeys

Add a few new tests to exercise the signal handler flow, especially with
PKEY 0 disabled:

 - Verify that the SIGSEGV handler is invoked when pkey 0 is disabled.

 - Verify that a thread which disables PKEY 0 segfaults with PKUERR when
   accessing the stack.

 - Verify that the SIGSEGV handler that uses an alternate signal stack is
   correctly invoked when the thread disabled PKEY 0

 - Verify that the PKRU value set by the application is correctly restored
   upon return from signal handling.

 - Verify that sigreturn() is able to restore the altstack even if the
   thread had PKEY 0 disabled

[ Aruna: Adapted to upstream ]
[ tglx: Made it actually compile. Restored protection_keys compile. Added
  	useful info to the changelog instead of bare function names. ]

Signed-off-by: Keith Lucas <[email protected]>
Signed-off-by: Aruna Ramakrishna <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

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