[compiler-rt] Fix signed integer overflow in int_mulo_impl.inc

[Ka-Ka]

Summary

When compiling compiler-rt with -fsanitize=undefined and running testcases you
end up with the following warning:

UBSan:/repo/uabkaka/llvm-project/compiler-rt/lib/builtins/int_mulo_impl.inc:24:23: signed integer overflow: -1 * -2147483648 cannot be represented in type 'si_int' (aka 'long')

This can be avoided by doing the multiplication in a matching unsigned variant
of the type.

This was found in an out of tree target.

[phosek]

LGTM

compiler-rt/lib/builtins/int_mulo_impl.inc

 
 // Effects: sets *overflow to 1  if a * b overflows
 
 static __inline fixint_t __muloXi4(fixint_t a, fixint_t b, int *overflow) {
   const int N = (int)(sizeof(fixint_t) * CHAR_BIT);
   const fixint_t MIN = (fixint_t)((fixuint_t)1 << (N - 1));
   const fixint_t MAX = ~MIN;
   *overflow = 0;
-  fixint_t result = a * b;
+  fixint_t result = (fixuint_t)a * b;
   if (a == MIN) {
     if (b != 0 && b != 1)
       *overflow = 1;
     return result;
   }
   if (b == MIN) {
     if (a != 0 && a != 1)
       *overflow = 1;