$(HOST_CC) is only used to invoke the preprocessor, and $(CC) can be
used instead now that there is a Tricore C compiler.  Remove the variable
from config-host.mak.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>