cad/openvsp: fix build on non-x86

Don't use amd64 code on all 64-bit architectures.
Include sys/param.h because of:
/usr/local/poudriere/ports/default/cad/openvsp/work/OpenVSP-OpenVSP_3.19.0/src/util/FileUtil.cpp:118:18: error: 'PATH_MAX' was not declared in this scope

118 |     char exepath[PATH_MAX];
    |                  ^~~~~~~~

PR: 242235
Approved by: fernape (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D22576