x86/microcode/amd: Cache builtin/initrd microcode early

There is no reason to scan builtin/initrd microcode on each AP.

Cache the builtin/initrd microcode in an early initcall so that the
early AP loader can utilize the cache.

The existing fs initcall which invoked save_microcode_in_initrd_amd() is
still required to maintain the initrd_gone flag. Rename it accordingly.
This will be removed once the AP loader code is converted to use the
cache.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231017211723.187566507@linutronix.de