microblaze: Add muldi3.c which contains routines for _muldi3

microblaze: Add muldi3.c which contains routines for _muldi3

Referenced arch/blackfin/lib/muldi3.c and the linux kernel.
Resolves issue seen when building u-boot for HW_MUL=0;

PLATFORM_CPPFLAGS += -mxl-soft-mul
PLATFORM_CPPFLAGS += -mno-xl-multiply-high

which resulted in error while linking to libgcc.a without mul hw (bs / m);

libgcc.a(_muldi3.o): In function `__muldi3':
.... src/gcc-4.6.2/libgcc/libgcc2.c:550: undefined reference to `_GLOBAL_OFFSET_TABLE_'

This link failure would not occur if we used gcc instead of ld directly, as
gcc will correctly use the crt's to resolve this link.

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