scudo: Use a table to look up the LSB for computing the odd/even mask. NFCI.

In the most common case we call computeOddEvenMaskForPointerMaybe()
from quarantineOrDeallocateChunk(), in which case we need to look up
the class size from the SizeClassMap in order to compute the LSB. Since
we need to do a lookup anyway, we may as well look up the LSB itself
and avoid computing it every time.

While here, switch to a slightly more efficient way of computing the
odd/even mask.

Differential Revision: https://reviews.llvm.org/D101018