x86/decompressor: Store boot_params pointer in callee save register

commit 8b63cba7 upstream.

Instead of pushing and popping %RSI several times to preserve the struct
boot_params pointer across the execution of the startup code, move it
into a callee save register before the first call into C, and copy it
back when needed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-8-ardb@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>