Makefile: HOSTCC set too late in clang builds

Currently we set HOSTCC=clang a little late meaning some minor bits
(utils/kconfig) are built with GCC. Move the assignment up the Makefile.

Change-Id: Ic72ad808eba0c0bf508bde34fb9bf0390c0b1d4d
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6140
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Idwer Vollering <vidwer@gmail.com>