m68k: libgcc: Fix lvalue abuse in umul_ppmm()

m68k: libgcc: Fix lvalue abuse in umul_ppmm()

    lib/muldi3.c:53:28: warning: asm output is not an lvalue
    lib/muldi3.c:53:28: warning: asm output is not an lvalue
    lib/muldi3.c:53:28: error: not addressable
    lib/muldi3.c:53:28: warning: generating address of non-lvalue (11)
    lib/muldi3.c:53:28: warning: generating address of non-lvalue (11)

Fix the lvalue warnings by replacing the casts on the output operands by
intermediate variables of the right type.

Fix the "not addressable" error by replacing the cast on the second
input operand by an intermediate variable, too.  Treat the other input
operand the same for consistency.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501030516.uZrwnuQQ-lkp@intel.com/
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/c408cfb85bfde8929dcaa4ebea29ade4e1452d8e.1736356696.git.geert@linux-m68k.org