[bug] Don't negate a signed number (UB fix)

* Fixes a potential UB in number to string conversion because of
    a possible undefined behavior caused by -int64_t(a) code. The
    fix replaces the code with Support::neg() function, which was
    designed for exactly this.

src/asmjit/core/string.cpp

@@ -291,7 +291,7 @@ Error String::_opNumber(ModifyOp op, uint64_t i, uint32_t base, size_t width, St
   // -----------
 
   if (Support::test(flags, StringFormatFlags::kSigned) && int64_t(i) < 0) {
-    i = uint64_t(-int64_t(i));
+    i = Support::neg(i);
     sign = '-';
   }
   else if (Support::test(flags, StringFormatFlags::kShowSign)) {