[sanitizer_common] Added 64-bit signed flag parser.

[hctim]

Summary

Adds a 64-bit signed flag parser for GWP-ASan's use.

[vitalybuka]

Test?

[hctim]

Added test. Still validating locally, but it can take a while...

[hctim]

Accidentally forgot to amend the commit before submitting, so the unit test was spliced off. It's in rL359632.

compiler-rt/lib/sanitizer_common/sanitizer_flag_parser.h

 inline bool FlagHandler<uptr>::Parse(const char *value) {
   const char *value_end;
   *t_ = internal_simple_strtoll(value, &value_end, 10);
   bool ok = *value_end == 0;
   if (!ok) Printf("ERROR: Invalid value for uptr option: '%s'\n", value);
   return ok;
 }
 
+template <>
+inline bool FlagHandler<s64>::Parse(const char *value) {
+  const char *value_end;
+  *t_ = internal_simple_strtoll(value, &value_end, 10);
+  bool ok = *value_end == 0;
+  if (!ok) Printf("ERROR: Invalid value for s64 option: '%s'\n", value);
+  return ok;
+}
+
 class FlagParser {
   static const int kMaxFlags = 200;
   struct Flag {
     const char *name;
     const char *desc;
     FlagHandlerBase *handler;
   } *flags_;
   int n_flags_;