x86/head_64: Store boot_params pointer in callee save register

Instead of pushing/popping %RSI to/from the stack every time a function
is called from startup_64(), store it in a callee preserved register
and grab it from there when its value is actually needed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-3-ardb@kernel.org