various cleanups to the Makefile

- remove unused OSREV varibale
- move various CROSS_COMPILE variables to the top and group them together
- move various PROC definitions to the top as a group
- remove usused MARCH variable
(issue #6892)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@17944 65c4cc65-6c06-0410-ace0-fbb531ad65f3