mk: override HOSTCC when overriding CC

mk: override HOSTCC when overriding CC

When overriding the CC variable on the commandline, the HOSTCC value
was remaining at "gcc", which caused errors on Free BSD10 when using a gcc
as e.g. gcc48, without a binary just called "gcc". This change overrides
HOSTCC when CC is overridden, which means that setting CC=gcc48, will
now also set HOSTCC=gcc48, allowing the "testhost" app to compile on
BSD10.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>