[builtins] Fix undefined behavior in negdi2.c and negti2.c

[Ka-Ka]

Summary

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

UBSan: negdi2.c:20:10: negation of -9223372036854775808 cannot be represented in type 'di_int' (aka 'long long'); cast to an unsigned type to negate this value to itself

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

The same kind of pattern is found in negti2.c

This was found in an out of tree target.

[MaskRay]

Looks great!

compiler-rt/lib/builtins/negdi2.c

 
 #include "int_lib.h"
 
 // Returns: -a
 
 COMPILER_RT_ABI di_int __negdi2(di_int a) {
   // Note: this routine is here for API compatibility; any sane compiler
   // should expand it inline.
-  return -a;
+  return -(du_int)a;
 }

compiler-rt/lib/builtins/negti2.c

 
 #ifdef CRT_HAS_128BIT
 
 // Returns: -a
 
 COMPILER_RT_ABI ti_int __negti2(ti_int a) {
   // Note: this routine is here for API compatibility; any sane compiler
   // should expand it inline.
-  return -a;
+  return -(tu_int)a;
 }
 
 #endif // CRT_HAS_128BIT