Fix net/haproxy and net/haproxy-devel build on ARM

USE_REGPARM=1 is specific for amd64 and i386. Conditionalize its definition and
let the port build on ARM

Approved by: demon (maintainer)
Obtained from: pfSense
MFH: 2017Q3
Sponsored by: Rubicon Communications, LLC (Netgate)
Differential Revision: https://reviews.freebsd.org/D11637