package/gcc: bump to version 8.3.0

Remove upsteam xtensa patches:
https://github.com/gcc-mirror/gcc/commit/68ca69a4854af36c90531b33a4c540464dbc6a23
https://github.com/gcc-mirror/gcc/commit/972057cb254e355805bfcd4a47d5c6f743cb76f4

Tested using Toolchain-builder:
https://gitlab.com/kubu93/toolchains-builder/pipelines/48904471



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>