nios2: fix div64 issue for gcc4

This patch fixes the run-time error on div64 when built with
gcc4, which was reported by jhwu0625 on nios forum. It merges
math support from libgcc of gcc4. This patch is copied from
nios2-linux.

It works with both gcc3 and gcc4. The old mult.c, divmod.c and
math.h are removed.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>