microblaze: Add __ucmpdi2() helper function

Add missing __ucmpdi2 helper function.

Error log:
kernel/built-in.o: In function `print_graph_duration':
: undefined reference to `__ucmpdi2'
kernel/built-in.o: In function `print_graph_duration':
: undefined reference to `__ucmpdi2'

Based on MIPS code.

Signed-off-by: Michal Simek <monstr@monstr.eu>