elf/dl-lookup.c: Move STB_GNU_UNIQUE handling to a function

Move handling of STB_GNU_UNIQUE symbols to a separate function
from do_lookup_x in order to make the code more readable.

The new function gets inlined with gcc 4.8 on ARM and the
do_lookup_x code becomes a few bytes smaller.

ChangeLog:

2014-06-23  Will Newton  <will.newton@linaro.org>

	* elf/dl-lookup.c (do_lookup_unique): New function.
	(do_lookup_x): Move STB_GNU_UNIQUE handling code
	to a separate function.