secyrity/softether-devel: Update to 4.37-9758-beta

This update reduces the occurrence of nonsensical packet ping-pong between
attackers targeting IPsec VPN devices with a wide range of global IP
addresses. See the release note and the announcement[1] for details.

While here, clean CFLAGS & LDFLAGS, sync ONLY_FOR_ARCHS with
security/softether-devel.

[1] https://ja.softether.org/9-about/news/901-210816

Relnotes: https://www.softether.org/5-download/history