When SMCCC_ARCH_WORKAROUND_1 is invoked from a lower EL running in
AArch32 state, ensure that the SMC call will take a shortcut in EL3.
This minimizes the time it takes to apply the mitigation in EL3.

When lower ELs run in AArch32, it is preferred that they execute the
`BPIALL` instruction to invalidate the BTB.  However, on some cores
the `BPIALL` instruction may be a no-op and thus would benefit from
making the SMCCC_ARCH_WORKAROUND_1 call go through the fast path.

Change-Id: Ia38abd92efe2c4b4a8efa7b70f260e43c5bda8a5
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>