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 <mingo@kernel.org>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: ricardo.neri@intel.com
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 <mingo@kernel.org>