x86/efi: Restore Firmware IDT before calling ExitBootServices()

Commit

  79419e13 ("x86/boot/compressed/64: Setup IDT in startup_32 boot path")

introduced an IDT into the 32-bit boot path of the decompressor stub.
But the IDT is set up before ExitBootServices() is called, and some UEFI
firmwares rely on their own IDT.

Save the firmware IDT on boot and restore it before calling into EFI
functions to fix boot failures introduced by above commit.

Fixes: 79419e13

 ("x86/boot/compressed/64: Setup IDT in startup_32 boot path")
Reported-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Cc: stable@vger.kernel.org # 5.13+
Link: https://lkml.kernel.org/r/20210820125703.32410-1-joro@8bytes.org