Abstract some more architecture-specific details away from SIMD functionality

Add a typedef to represent vectors containing four 32-bit integers,
and add functions operating on them. Also separate out saturating
subtraction into its own function. The motivation for this is to
prepare for a future commit to add ARM NEON support.

Nathan Bossart

Reviewed by John Naylor and Tom Lane
Discussion: https://www.postgresql.org/message-id/flat/CAFBsxsEyR9JkfbPcDXBRYEfdfC__OkwVGdwEAgY4Rv0cvw35EA%40mail.gmail.com#aba7a64b11503494ffd8dd27067626a9
