x86: Simplify _IntVecAlloc() logic

Instead of using logic choosing between invoking find_lsb_set() and
find_msb_set(), mask out the unwanted bits and always invoke find_lsb_set().
This also has the side benefit of simplifying the error checking when DEBUG is
enabled.

Change-Id: I7f3e529ab3807b4433291197793e79e371d06ae3
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>