Force 32-bit math in abs()

In AVR the labs() function takes a 32-bit signed int parameter. On
the PC it's at least 64-bits and maybe more.  When we have a 32-bit
unsigned value we're taking the labs() of, coercing it to 32-bits
first turns our high-bit into a sign, but coercing it to 64-bits
does not.  This causes all our negative values to appear to be
really big positive ones.

Create a new function abs32() which always coerces its argument to
a int32_t first before return the abs value.  Use that function
whereved needed in dda.c.

This fixes a problem on the simulator which caused negative
direction movements to "never" end.