Fix 'Copyright (C) 2020-21' into '2020-2021'

Fix 'Copyright (C) 2020-21' into '2020-2021'

'contrib/update-copyright.py' currently complains:

    gcc/config/rs6000/rs6000-gen-builtins.c: unrecognised year: 21
    gcc/config/rs6000/rs6000-overload.def: unrecognised year: 21
    gcc/config/rs6000/rbtree.h: unrecognised year: 21
    gcc/config/rs6000/rbtree.c: unrecognised year: 21
    gcc/config/rs6000/rs6000-builtin-new.def: unrecognised year: 21

Fix up files added in commit fa5f8b49e55caf5bb341f5eb6b5ab828b9286425
"rs6000: Red-black tree implementation for balanced tree search",
commit 4a720a9547320699aceda7d2e0b08de5ab40132f
"rs6000: Add initial input files",
commit bd5b625228d545d5ecb35df24f9f094edc95e3fa
"rs6000: Initial create of rs6000-gen-builtins.c".

	gcc/
	* config/rs6000/rbtree.c: Fix 'Copyright (C) 2020-21' into '2020-2021'
	* config/rs6000/rbtree.h: Likewise.
	* config/rs6000/rs6000-builtin-new.def: Likewise.
	* config/rs6000/rs6000-gen-builtins.c: Likewise.
	* config/rs6000/rs6000-overload.def: Likewise.