toolchain/gcc: switch to version 11 by default

toolchain/gcc: switch to version 11 by default

gcc10 seem to increase build size and gcc11 seem to fix that.

Compile tests:
* all

Runtime tests:
* ath79
* mpx85xx/p2020
* mvebu
* x86/64

Special thanks to Rosen for fixing layerscape & bcm63xx

Signed-off-by: Paul Spooren <mail@aparcar.org>
Reviewed-by: Rui Salvaterra <rsalvaterra@gmail.com
Acked-by: Rosen Penev <rosenp@gmail.com>
Acked-by: Rui Salvaterra <rsalvaterra@gmail.com
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Aleksander Jan Bajkowski <A.Bajkowski@stud.elka.pw.edu.pl>
Tested-by: Pawel Dembicki <paweldembicki@gmail.com>
Tested-by: Aleksander Jan Bajkowski <A.Bajkowski@stud.elka.pw.edu.pl>