Changing the register used for the IPC buffer is useful as the standard TLS model for
aarch64 is to use tpidr_el0 for the thread pointer, which we are presently using
for the IPC buffer. This therefore paves the way for implementing proper TLS support
on aarch64.
Since we are not using tpidr_el0 for the IPC buffer we need to save/restore (or at
least clear) this register on thread switch to prevent it being a user channel. Saving
and restoring of it is achieved by ensuring CONFIG_IPC_BUF_TPIDRURW is not set, which
as we are no longer using this strategy this will be the case. To that end as aarch64
only has one IPC buffer strategy this commit makes the choice of IPC buffer strategy
dependeont on aarch32.
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