Mk/bsd.port.mk: Add MACHINE_ARCH to WRK_ENV

make(1) has a built-in variable MACHINE_ARCH that defaults to the value
of sysctl hw.machine_arch. This is not correct when building packages
under qemu. In that case poudriere sets MACHINE_ARCH in its make.conf.
Since commit d7511b9d0074 make.conf is not picked up by make during
build so pass MACHINE_ARCH through the environment.

This affects building kmod ports under qemu.

This is a workaround for make not looking at UNAME_p when setting
MACHINE_ARCH. It does look at UNAME_m when setting MACHINE.

PR: 281674
Reported by: jrtc27
Exp-run by: antoine
Approved by: portmgr (antoine)