x86/lib/copy_page_64.S: Use generic ALTERNATIVE macro

x86/lib/copy_page_64.S: Use generic ALTERNATIVE macro

... instead of the semi-version with the spelled out sections.

What is more, make the REP_GOOD version be the default copy_page()
version as the majority of the relevant x86 CPUs do set
X86_FEATURE_REP_GOOD. Thus, copy_page gets compiled to:

  ffffffff8130af80 <copy_page>:
  ffffffff8130af80:       e9 0b 00 00 00          jmpq   ffffffff8130af90 <copy_page_regs>
  ffffffff8130af85:       b9 00 02 00 00          mov    $0x200,%ecx
  ffffffff8130af8a:       f3 48 a5                rep movsq %ds:(%rsi),%es:(%rdi)
  ffffffff8130af8d:       c3                      retq
  ffffffff8130af8e:       66 90                   xchg   %ax,%ax

  ffffffff8130af90 <copy_page_regs>:
  ...

and after the alternatives have run, the JMP to the old, unrolled
version gets NOPed out:

  ffffffff8130af80 <copy_page>:
  ffffffff8130af80:  66 66 90		xchg   %ax,%ax
  ffffffff8130af83:  66 90		xchg   %ax,%ax
  ffffffff8130af85:  b9 00 02 00 00	mov    $0x200,%ecx
  ffffffff8130af8a:  f3 48 a5		rep movsq %ds:(%rsi),%es:(%rdi)
  ffffffff8130af8d:  c3			retq

On modern uarches, those NOPs are cheaper than the unconditional JMP
previously.

Signed-off-by: Borislav Petkov <bp@suse.de>