Code owner for the XCore backend

Hello all,

As I’ve left XMOS, I think it’s best if I transfer ownership of the XCore backend. Robert Lytton has graciously volunteered to be the new code owner, subject to community approval.

Over the past few years, Robert has contributed many improvements and bugfixes to the XCore backend as well as being responsible for adding the XCore target to clang. I therefore believe he is an obvious choice to take over code ownership.

If there’s no objections we’ll update CODE_OWNERS.txt in a few days.

SGTM, thank you for your contributions Richard, and congrats to Robert!