monitor/target-ppc: Define target_get_monitor_def

monitor/target-ppc: Define target_get_monitor_def

At the moment get_monitor_def() returns only registers from statically
defined monitor_defs array. However there is a lot of BOOK3S SPRs
which are not in the list and cannot be printed from the monitor.

This adds a new target platform hook - target_get_monitor_def().
The hook is called if a register was not found in the static
array returned by the target_monitor_defs() hook.

The hook is only defined for POWERPC, it returns registered
SPRs and fails on unregistered ones providing the user with information
on what is actually supported on the running CPU. The register value is
saved as uint64_t as it is the biggest supported register size;
target_ulong cannot be used because of the stub - it is in a "common"
code and cannot include "cpu.h", etc; this is also why the hook prototype
is redefined in the stub instead of being included from some header.

This replaces static descriptors for GPRs, FPRs, SRs with a helper which
looks for a value in a corresponding array in the CPUPPCState.
The immediate effect is that all 32 SRs can be printed now (instead of 16);
later this can be reused for VSX or TM registers.

This replaces callbacks for MSR and XER with static descriptors in
monitor_defs as they are stored in CPUPPCState.

While we are here, this adds "cr" as a synonym of "ccr".

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>