x86, mm: Clean up and simplify NX enablement

The 32- and 64-bit code used very different mechanisms for enabling
NX, but even the 32-bit code was enabling NX in head_32.S if it is
available.  Furthermore, we had a bewildering collection of tests for
the available of NX.

This patch:

a) merges the 32-bit set_nx() and the 64-bit check_efer() function
   into a single x86_configure_nx() function.  EFER control is left
   to the head code.

b) eliminates the nx_enabled variable entirely.  Things that need to
   test for NX enablement can verify __supported_pte_mask directly,
   and cpu_has_nx gives the supported status of NX.

Signed-off-by: H. Peter Anvin <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Vegard Nossum <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Cc: Chris Wright <[email protected]>
LKML-Reference: <[email protected]>
Acked-by: Kees Cook <[email protected]>

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