Stop gap fix for __attribute__((cold)) compiler bug in MinGW 8.1

The buildfarm animal walleye, running MinGW 8.1 has been having problems
ever since 697e1d02f and 913ec71d6 went in.  This appears to be a bug in
assembler which was fixed in a later version.

For now, in order to get that animal running green again, let's just
define pg_attribute_cold and pg_attribute_hot to be empty macros on that
compiler.  Hopefully, we can get the support of the owner of the animal to
upgrade to a less buggy compiler and revert this at a later date.

Discussion: https://postgr.es/m/286560.1606233316@sss.pgh.pa.us

src/include/c.h

@@ -200,6 +200,21 @@
 #define pg_noinline
 #endif
 
+/*
+ * For now, just define pg_attribute_cold and pg_attribute_hot to be empty
+ * macros on minGW 8.1.  There appears to be a compiler bug that results in
+ * compilation failure.  At this time, we still have at least one buildfarm
+ * animal running that compiler, so this should make that green again. It's
+ * likely this compiler is not popular enough to warrant keeping this code
+ * around forever, so let's just remove it once the last buildfarm animal
+ * upgrades.
+ */
+#if defined(__MINGW64__) && __GNUC__ == 8 && __GNUC_MINOR__ == 1
+
+#define pg_attribute_cold
+#define pg_attribute_hot
+
+#else
 /*
  * Marking certain functions as "hot" or "cold" can be useful to assist the
  * compiler in arranging the assembly code in a more efficient way.
@@ -216,6 +231,8 @@
 #define pg_attribute_hot
 #endif
 
+#endif							/* defined(__MINGW64__) && __GNUC__ == 8 &&
+								 * __GNUC_MINOR__ == 1 */
 /*
  * Mark a point as unreachable in a portable fashion.  This should preferably
  * be something that the compiler understands, to aid code generation.