Improved gcc to inject nptl cross-compile tweaks through configcache

Note: libc_cv_forced_unwind=yes and libc_cv_c_cleanup=yes" config cache
      variables are needed for all archs if TLS is enabled, and the
      config cache approach seems to be more reliable than patching!

base/glibc/glibc.conf

@@ -66,6 +66,11 @@ if [ $SDECFG_PKG_GLIBC_TLS = 1 ]; then
 	var_append glibc_addons "," "nptl"
 	var_append extraconfopt " " "--with-tls"
 	var_append extraconfopt " " "--with-__thread"
+
+	if atstage cross ; then
+		var_append configcache ' ' "libc_cv_forced_unwind=yes"
+		var_append configcache ' ' "libc_cv_c_cleanup=yes"
+	fi
 else    
 	var_append glibc_addons "," "linuxthreads"
 	var_append extraconfopt " " "--without-__thread"