databases/percona57-*: Fix build for i386
Make build-script assume that FreeBSD's Clang doesn't
have "atomic_*" functions builtin implemented on i386
While here, allow to build on DragonFly BSD and
drop "-fabi-version=2" from C/CXX flags
Reported by: pkg-fallout
MFH: 2020Q1
Fix build on gcc-based architectures by removing the assumption that FreeBSD
must imply clang. While here, remove duplicate definition from USES.
PR: 234580
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)