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

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