openrisc: export __mulsi3 for modules

OpenRISC configurations without hardware multiplication fail to build
modules which contain compiler-generated calls to __mulsi3.  The failure
is widespread, affecting filesystems, drivers and KUnit tests at modpost.

When CONFIG_OPENRISC_HAVE_INST_MUL is disabled, the architecture Makefile
passes -msoft-mul to GCC.  The multiplication helper is supplied by the
libgcc archive linked into vmlinux, but or32_ksyms.c does not export it. 
Built-in callers can therefore link while module callers cannot resolve
the symbol.

Export __mulsi3 alongside the other compiler-generated helpers.

The omission predates 7c3f4d699f ("resource, kunit: stop selecting
GET_FREE_REGION"), which the robot identified when reporting an additional
failing module.  It was already present in the initial OpenRISC export
list.

Fixes: 616257669e ("OpenRISC: Miscellaneous")
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202610290106.o8Fs3NvO-lkp@intel.com/
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Karl Mehltretter <kmehltretter@gmail.com>
Cc: <stable@vger.kernel.org>

arch/openrisc/kernel/or32_ksyms.c

@@ -35,6 +35,7 @@ DECLARE_EXPORT(__divsi3);
 DECLARE_EXPORT(__umodsi3);
 DECLARE_EXPORT(__modsi3);
 DECLARE_EXPORT(__muldi3);
+DECLARE_EXPORT(__mulsi3);
 DECLARE_EXPORT(__ashrdi3);
 DECLARE_EXPORT(__ashldi3);
 DECLARE_EXPORT(__lshrdi3);