Make EPICS_HOST_ARCH available to hook scripts

[mdavidsaver]

I would find it helpful to have $EPICS_HOST_ARCH available in *_HOOK scripts. eg. to write configure/CONFIG_SITE.Common.$(EPICS_HOST_ARCH).

I am not quite sure of the best way/place to add this. Maybe as simple as moving detect_epics_host_arch() before add_dependency() in prepare()?

[ralphlange]

Not used hook scripts a lot so far, but I can imagine the usefulness.