x86: trivial printk optimizations

In arch/x86/boot/printf.c gets rid of unused tail of digits: const char
*digits = "0123456789abcdefghijklmnopqrstuvwxyz"; (we are using 0-9a-f
only)

Uses smaller/faster lowercasing (by ORing with 0x20)
if we know that we work on numbers/digits. Makes
strtoul smaller, and also we are getting rid of

  static const char small_digits[] = "0123456789abcdefx";
  static const char large_digits[] = "0123456789ABCDEFX";

since this works equally well:

  static const char digits[16] = "0123456789ABCDEF";

Size savings:

$ size vmlinux.org vmlinux
   text    data     bss     dec     hex filename
 877320  112252   90112 1079684  107984 vmlinux.org
 877048  112252   90112 1079412  107874 vmlinux

It may be also a tiny bit faster because code has less
branches now, but I doubt it is measurable.

[ hugh@veritas.com: uppercase pointers fix ]

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>