archivers/snappy: fix build on armv7

A recent commit introduced NEON intrinsics that are only available on
aarch64. Work around this problem by adding polyfill for the missing
intrinsic.

Approved by: portmgr (build fix blanket)
Obtained from: https://stackoverflow.com/a/70154591/417501
See also: https://groups.google.com/g/snappy-compression/c/m1ZNPsobiQs
MFH: 2023Q2

(cherry picked from commit 9a0a2422622d5feee7d77ddc954540daff449a1d)