x86/boot: Move the LA57 trampoline to separate source file

To permit the EFI stub to call this code even when building the kernel
without the legacy decompressor, move the trampoline out of the latter's
startup code.

This is part of an ongoing WIP effort on my part to make the existing,
generic EFI zboot format work on x86 as well.

No functional change intended.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250313120324.1095968-2-ardb+git@google.com