[builtins] Fix signed shift overflows in absvti2.c and negvti2.c

[Ka-Ka]

Summary

When compiling the builtins with the undefined behavior sanitizer and running testcases you end up with the following warning:

UBSan: negvti2.c:22:32: left shift of 1 by 127 places cannot be represented in type 'ti_int' (aka 'int128')
UBSan: absvti2.c:23:23: left shift of 1 by 127 places cannot be represented in type 'ti_int' (aka 'int128')

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

This is the same kind of fixes that already was done in commit
854686f0794b9d0695d5a0a85ea1e7e71ba8edfd

This was found in an out of tree target.

compiler-rt/lib/builtins/absvti2.c

 #ifdef CRT_HAS_128BIT
 
 // Returns: absolute value
 
 // Effects: aborts if abs(x) < 0
 
 COMPILER_RT_ABI ti_int __absvti2(ti_int a) {
   const int N = (int)(sizeof(ti_int) * CHAR_BIT);
-  if (a == ((ti_int)1 << (N - 1)))
+  if (a == (ti_int)((tu_int)1 << (N - 1)))
     compilerrt_abort();
   const ti_int s = a >> (N - 1);
   return (a ^ s) - s;
 }
 
 #endif // CRT_HAS_128BIT

compiler-rt/lib/builtins/negvti2.c

 
 #ifdef CRT_HAS_128BIT
 
 // Returns: -a
 
 // Effects: aborts if -a overflows
 
 COMPILER_RT_ABI ti_int __negvti2(ti_int a) {
-  const ti_int MIN = (ti_int)1 << ((int)(sizeof(ti_int) * CHAR_BIT) - 1);
+  const ti_int MIN = (tu_int)1 << ((int)(sizeof(ti_int) * CHAR_BIT) - 1);
   if (a == MIN)
     compilerrt_abort();
   return -a;
 }
 
 #endif // CRT_HAS_128BIT