Fix unsigned_abs function on Unix (#8805)

Fix unsigned_abs function on Unix (#8805)

The unsigned_abs function in jit.h is implemented using labs function.
However labs has 32 bit argument / result, while the unsigned_abs expects
it to be 64 bit.
The correct function to use for FEATURE_PAL is llabs. But since we now have
abs overload for __int64 in pal.h, we can just remove the #ifdef for FEATURE_PAL
and use the same implementation for both Unix and Windows.