The SPSR register is named within the Unicorn headers, but the code
to access it is absent. This means that it will always read as 0 and
ignore writes. This makes it harder to work with changes in processor
mode, as the usual way to return from a CPU exception is a
`MOVS pc, lr` for undefined instructions or `SUBS pc, lr, #4`
for most other aborts - which implicitly restores the CPSR from SPSR.
This change adds the access to the SPSR so that it can be read and
written as the caller might expect.
99097cab4c