Ensure that HOSTCC and HOSTCFLAGS are set for DejaGNU

Some tests from GCC test suite require HOST{CC,CFLAGS} variables. However
site.exp generated by contrib/test_installed scripts doesn't contain those
variables. To avoid errors we need to define them in our site.exp. Test
suite run from the "make" instead of test_installed will not be affected,
because our site.exp will not overwrite variables if they are defined.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>