Fix Math.abs stub bug.

  the expected result should be in SMI format, so we need to tag the INT32
to SMI when returning to caller.
Fix logical right shift by 0 bug.

  When doing logical right shift by 0, we may produce a positive number
from a negative number, and the positive number will be out of int32
range, so we need to convert the result into double float.
Remove "if 0" guarded code.