microblaze: Change libgcc-style functions from lib-y to obj-y

Following precedence set by MIPs:
"[MIPS] Change libgcc-style functions from lib-y to obj-y"
(sha1: f7c27781),
switch the goal definition for kbuild to obj-y to ensure
object files linked in vmlinux if only modules were users
of these functions.

Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>