x86/setup: Move duplicate boot_cpu_data definition out of the ifdeffery

Both the if and else blocks define an exact same boot_cpu_data variable, move
the duplicate variable definition out of the if/else block.

In addition, do some other minor cleanups.

  [ bp: Massage. ]

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20220601122914.820890-1-ytcoode@gmail.com