x86/percpu: Clean up <asm/percpu.h> a bit

 - Fix misc typos

 - There's 4 variants of the same spelling right now:

     'per-CPU', 'per CPU', 'percpu' and 'per-cpu'

   Standardize on 'per-CPU' only.

 - s/makes gcc load
    /makes the compiler load

 - Instead of:

     #ifdef CONFIG_XXXX
     #define YYYY FOO
     #else
     #define YYYY BAR
     #endif

   Use the slightly more readable form of:

     #ifdef CONFIG_XXXX
     # define YYYY FOO
     #else
     # define YYYY BAR
     #endif

 - Standardize & expand '#else' and '#endif' comments

 - Fix comment style

 - Capitalize x86 instruction names in comments

No change in code.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org