Consistently use -std=c++2a, not -std=c++20

Commit 63a911e657 switched
to `-std=c++20` since at the time we required GCC 10 or higher.

Commit d5ce5329ea partially reverted
to `-std=c++2a` since we once again support GCC 9, which did not
yet handle `std=c++20`.

Makefile

@@ -257,7 +257,7 @@ tidy: src/asm/parser.hpp src/link/script.hpp
 iwyu:
 	$Qenv ${MAKE} \
 		CXX="include-what-you-use" \
-		REALCXXFLAGS="-std=c++20 -I include"
+		REALCXXFLAGS="-std=c++2a -I include"
 
 # Target used in development to conveniently invoke RGBDS binaries with Wine.
 wine-shim: