Makefile: Fix HOSTCC for clang

Change-Id: I0470cf2c1e5266ceb33d55c9d616bfff37e97ed6
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7272
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>