x86/cpu: Use alternative to generate the TASK_SIZE_MAX constant

We used to generate this constant with static jumps, which certainly
works, but generates some quite unreadable and horrid code, and extra
jumps.

It's actually much simpler to just use our alternative_asm()
infrastructure to generate a simple alternative constant, making the
generated code much more obvious (and straight-line rather than "jump
around to load the right constant").

Acked-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>