Don't use the uint128_t P-256 code under MSAN.

MSAN appears to have a bug that causes this code to be miscompiled when
compiled with optimisations. In order to prevent that bug from holding
everything up, this change disables that code when MEMORY_SANITIZER is
defined. The generic elliptic-curve code can pick up the slack in that
case.

Change-Id: I7ce26969b3ee0bc0b0496506f06a8cf9b2523cfa