configure: use cc, not host_cc to set cross_cc for build arch

Otherwise you run into hilarity like trying when cross compiling a 32
bit ARM build on a 64 bit system trying to use host_cc to build 32 bit
test cases.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210512102051.12134-32-alex.bennee@linaro.org>

configure

@@ -1651,7 +1651,7 @@ case "$cpu" in
     # No special flags required for other host CPUs
 esac
 
-eval "cross_cc_${cpu}=\$host_cc"
+eval "cross_cc_${cpu}=\$cc"
 cross_cc_vars="$cross_cc_vars cross_cc_${cpu}"
 QEMU_CFLAGS="$CPU_CFLAGS $QEMU_CFLAGS"