Makefile: Update compiler flags organization
Makefile: Update compiler flags organization

This allows CFLAGS to be exported as an environment variable and not
overrided by the Makefile.

This also allows ARCH_CFLAGS to be set in order to clear
-mtune=generic, which does not exist on gcc for some
platforms (e.g. s390x)

cemeyer/hadoofus,cemeyer/hadoofus