x86_64: Remove pointless set_64bit() usage

The use of set_64bit() in X86_64 only code is pretty pointless, seeing
how it's a direct assignment. Remove all this nonsense.

[nathanchance: unbreak irte]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221022114425.168036718%40infradead.org