x86/umip: Select X86_INTEL_UMIP by default

x86/umip: Select X86_INTEL_UMIP by default

UMIP does cause any performance penalty to the vast majority of x86 code
that does not use the legacy instructions affected by UMIP.

Also describe UMIP more accurately and explain the behavior that can be
expected by the (few) applications that use the affected instructions.

Suggested-by: Ingo Molnar <[email protected]>
Signed-off-by: Ricardo Neri <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/1510640985-18412-2-git-send-email-ricardo.neri-calderon@linux.intel.com
[ Spelling fixes, rewrote the changelog. ]
Signed-off-by: Ingo Molnar <[email protected]>