Examine MACHINE_ARCH instead of ARCH with respect to installation
commands that are dependent on the host machine architecture.
This fixes installation failure on amd64 in some circumstances.
PR: ports/80679 , ports/90252 
Submitted by: OMATSU Shinichiro <firstname.lastname@example.org> 
Torfinn Ingolfsen <email@example.com> 
Approved by: emulation (maintainer, timeout 7 months)