Fix resume when core state has been modified

Fix resume when core state has been modified

Sometimes it is necessary to resume into a different state (ARM/Thumb)
than at debug state entry. According to the documentation this should
be possible with "arm core_state arm|thumb" before the resume command,
however the original code also restores the original CPSR, which
overrides whatever state the core was set to. This seems to work on some
cores (e.g. Cortex-A5) but not on others (e.g. Cortex-A9). Using the "BX"
instruction to set resume PC and core state works on Cortex-A9 and
ARM11, but is not sufficient on Cortex-A5, where an explicit write to
the PC (MOV pc, r0) is required additionally.

Change-Id: Ic03153b4b250fbb8cf6c75f8e329fb34829aa35f
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3386
Tested-by: jenkins
Reviewed-by: Alexander Stein <alexanders83@web.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>