cmd/syncthing: Add selectable sha256 package (fixes #3613, fixes #3614)

This adds autodetection of the fastest hashing library on startup, thus
handling the performance regression. It also adds an environment
variable to control the selection, STHASHING=standard (Go standard
library version, avoids SIGILL crash when the minio library has bugs on
odd CPUs), STHASHING=minio (to force using the minio version) or unset
for the default autodetection.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3617
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