Support SSE2 intrinsics where available

Support SSE2 intrinsics where available

SSE2 vector instructions are part of the spec for the 64-bit x86
architecture. Until now we have relied on the compiler to autovectorize
in some limited situations, but some useful coding idioms can only be
expressed explicitly via compiler intrinsics. To this end, add a header
that defines USE_SSE2 where available. While x86-only for now, we can
add other architectures in the future. This will also be the intended
place for helper functions that use vector operations.

Reviewed by Nathan Bossart and Masahiko Sawada

Discussion: https://www.postgresql.org/message-id/CAFBsxsE2G_H_5Wbw%2BNOPm70-BK4xxKf86-mRzY%3DL2sLoQqM%2B-Q%40mail.gmail.com