gdbstub: Synchronize CPU state unconditionally in gdb_set_cpu_pc

gdbstub: Synchronize CPU state unconditionally in gdb_set_cpu_pc

Synchronize the CPU state via cpu_sychronize_state() unconditionally
in gdb_set_cpu_pc() rather than only in some of the target ifdef
ladder cases.

We can divide the CPUs into three categories:
 * non-KVM targets: no change of behaviour since we will use the
   kvm-stub.c no-op function.
 * i386 and s390: no change of behaviour since they were already
   calling this function
 * PPC (in KVM mode): this fixes an error: failing to synchronise
   was accidental and probably a bug.

This also paves the way for other targets (specifically ARM) which
can add KVM support in future without having to add another target
specific change to this bit of code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>