efi/libstub/x86: Deal with exit() boot service returning

Even though it is uncommon, there are cases where the Exit() EFI boot
service might return, e.g., when we were booted via the EFI handover
protocol from OVMF and the kernel image was specified on the command
line, in which case Exit() attempts to terminate the boot manager,
which is not an EFI application itself.

So let's drop into an infinite loop instead of randomly executing code
that isn't expecting it.

Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
[ardb: put 'hlt' in deadloop]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200303080648.21427-1-ardb@kernel.org
Link: https://lore.kernel.org/r/20200308080859.21568-15-ardb@kernel.org