OF: Add a separate direct kernel loading word

Currently, go-64 is used for booting a kernel from qemu (i.e. -kernel).
However, there is an expectation from users that this should be able to
boot not just vmlinux kernels but things like Zimages too.

The bootwrapper of a BE zImage is a 32-bit ELF. Attempting to load that
with go-64 means that it will be ran with MSR_SF set (64-bit mode). This
crashes early in boot (usually due to what should be 32-bit operations
being done with 64-bit registers eventually leading to an incorrect
address being generated and branched to).

Note that our 64-bit payloads are prepared to enter with MSR_SF cleared
and set it themselves very early.

Add a new word named go-direct that will execute any simple payload
in-place and will enter with MSR_SF cleared. This allows booting a BE
zImage from qemu with -machine kernel-addr=0.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

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