databases/mongodb50: enable non-AVX CPUs by default

By popular demand as this port is used by net-mgmt/unifi8.

I couldn't find any information on the internet that the binary really
does something which requires AVX. For example this discussion:
https://github.com/turnkeylinux/tracker/issues/1724

(cherry picked from commit 1707c0aac83aaf892c07bfc05573d1efa8ec5724)

PR: 277990
Requested by: glebius@