stdlib: Simplify buffer management in canonicalize

stdlib: Simplify buffer management in canonicalize

Move the buffer management from realpath_stk to __realpath.  This
allows returning directly after allocation errors.

Always make a copy of the result buffer using strdup even if it is
already heap-allocated.  (Heap-allocated buffers are somewhat rare.)
This avoids GCC warnings at certain optimization levels.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit ef0700004bf0dccf493a5e8e21f71d9e7972ea9f)