configure: don't enable cross compilers unless in target_list

This avoids the unfortunate effect of always builds the pc-bios blobs
for targets the user isn't interested in.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20221027183637.2772968-5-alex.bennee@linaro.org>

configure

@@ -1877,6 +1877,15 @@ probe_target_compiler() {
   container_cross_ranlib=
   container_cross_strip=
 
+  # We shall skip configuring the target compiler if the user didn't
+  # bother enabling an appropriate guest. This avoids building
+  # extraneous firmware images and tests.
+  if test "${target_list#*$1}" != "$1"; then
+      break;
+  else
+      return 1
+  fi
+
   target_arch=${1%%-*}
   case $target_arch in
     aarch64) container_hosts="x86_64 aarch64" ;;