Update comment about __sync_lock_test_and_set() bug.

State the earliest known fixed version, so we can someday judge the
workaround to be obsolete.

src/include/port/atomics.h

@@ -89,7 +89,7 @@
 /*
  * gcc or compatible, including clang and icc.  Exclude xlc.  The ppc64le "IBM
  * XL C/C++ for Linux, V13.1.2" emulates gcc, but __sync_lock_test_and_set()
- * of one-byte types elicits SIGSEGV.
+ * of one-byte types elicits SIGSEGV.  That bug was gone by V13.1.5 (2016-12).
  */
 #if (defined(__GNUC__) || defined(__INTEL_COMPILER)) && !(defined(__IBMC__) || defined(__IBMCPP__))
 #include "port/atomics/generic-gcc.h"