Add a special case for handling builtin_mul_overflow with unsigned
inputs and a signed output to avoid emitting the muloti4 library
call on x86_64. muloti4 is not implemented in libgcc, so avoiding
this call fixes compilation of some programs that call
builtin_mul_overflow with these arguments.
For example, this fixes the build of cpio with clang, which includes code from
gnulib that calls __builtin_mul_overflow with these argument types.