libgcc_s: expose __divmoddi4 on i386

GCC has used this for some time (since 7.0) and apparently we were
getting away with using the hidden symbol, but when linking with
--no-undefined-version we get an error unless it's properly exported.
(For anyone who wonders at the assymetry, __udivmoddi4 is indeed much
older and was introduced with GCC 3.0.)

MFC after:	3 days
Reviewed by:	dim
Differential Revision:	https://reviews.freebsd.org/D44878

lib/libgcc_s/Symbol.map

@@ -221,3 +221,9 @@ GCC_4.6.0 {
 	__unordtf2;
 };
 #endif
+
+GCC_7.0.0 {
+#ifdef __i386__
+	__divmoddi4;
+#endif
+};

lib/libgcc_s/Versions.def

@@ -31,3 +31,6 @@ GCC_4.3.0 {
 
 GCC_4.6.0 {
 } GCC_4.3.0;
+
+GCC_7.0.0 {
+} GCC_4.6.0;