Makefile: default to cc for host compiler

Since the host compiler might not be gcc but e.g. clang
default to cc/c++ to invoke it.

cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
cc: Tom Rini <trini@ti.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>