x86/entry/fred: Push __KERNEL_CS directly

Push __KERNEL_CS directly, rather than moving it into RAX and then
pushing RAX.

No functional changes.

Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250822071644.1405268-1-xin@zytor.com