deprecation: don't enable TCG plugins by default on 32 bit hosts

The existing plugins already liberally use host pointer stuffing for
passing user data which will fail when doing 64 bit guests on 32 bit
hosts. We should discourage this by officially deprecating support and
adding another nail to the 32 bit host coffin.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240916085400.1046925-2-alex.bennee@linaro.org>

configure

@@ -516,6 +516,25 @@ case "$cpu" in
     ;;
 esac
 
+# Now we have our CPU_CFLAGS we can check if we are targeting a 32 or
+# 64 bit host.
+
+check_64bit_host() {
+cat > $TMPC <<EOF
+#if __SIZEOF_POINTER__ != 8
+#error not 64 bit system
+#endif
+int main(void) { return 0; }
+EOF
+  compile_object "$1"
+}
+
+if check_64bit_host "$CPU_CFLAGS"; then
+    host_bits=64
+else
+    host_bits=32
+fi
+
 if test -n "$host_arch" && {
     ! test -d "$source_path/linux-user/include/host/$host_arch" ||
     ! test -d "$source_path/common-user/host/$host_arch"; }; then
@@ -1028,7 +1047,7 @@ if test "$static" = "yes" ; then
   fi
   plugins="no"
 fi
-if test "$plugins" != "no"; then
+if test "$plugins" != "no" && test $host_bits -eq 64; then
   plugins=yes
   subdirs="$subdirs contrib/plugins"
 fi