After Go 1.21 is out, build with PGO enabled

[benhoyt]

Profile-guided optimization (PGO) will become ready for general use with the release of Go 1.21.

From a quick test with go1.21rc3, GoAWK built with Go 1.21 (without PGO) is already significantly faster than GoAWK built with Go 1.20. So that's a good start. But enabling PGO looks like it improves the performance a bit more.

Will need to determine what set of representative benchmark(s) we run to produce the PGO profile.

[benhoyt]

I did this in 5311ab1, closing.