configure: move detected gdb to TCG's config-host.mak

When tests/tcg gained it's own config-host.mak we forgot to move the
GDB detection.

Fixes: 544f4a2578 (tests/tcg: isolate from QEMU's config-host.mak)
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-6-alex.bennee@linaro.org>

configure

@@ -2370,6 +2370,8 @@ if test -n "$gdb_bin"; then
     gdb_version=$($gdb_bin --version | head -n 1)
     if version_ge ${gdb_version##* } 9.1; then
         echo "HAVE_GDB_BIN=$gdb_bin" >> $config_host_mak
+    else
+        gdb_bin=""
     fi
 fi
 
@@ -2453,6 +2455,11 @@ echo "# Automatically generated by configure - do not modify" > $config_host_mak
 echo "SRC_PATH=$source_path" >> $config_host_mak
 echo "HOST_CC=$host_cc" >> $config_host_mak
 
+# versioned checked in the main config_host.mak above
+if test -n "$gdb_bin"; then
+    echo "HAVE_GDB_BIN=$gdb_bin" >> $config_host_mak
+fi
+
 tcg_tests_targets=
 for target in $target_list; do
   arch=${target%%-*}