microblaze: lib: Add lib function declarations

Function declarations fix these sparse warnings:
arch/microblaze/lib/ashldi3.c:5:11: warning: symbol
	'__ashldi3' was not declared. Should it be static?
arch/microblaze/lib/muldi3.c:50:8: warning: symbol
	'__muldi3' was not declared. Should it be static?
arch/microblaze/lib/cmpdi2.c:5:11: warning: symbol
	'__cmpdi2' was not declared. Should it be static?
arch/microblaze/lib/lshrdi3.c:5:11: warning: symbol
	'__lshrdi3' was not declared. Should it be static?
arch/microblaze/lib/ashrdi3.c:5:11: warning: symbol
	'__ashrdi3' was not declared. Should it be static?
arch/microblaze/lib/ucmpdi2.c:5:11: warning: symbol
	'__ucmpdi2' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>