pyOCD/gdbserver: Fixed XPSR variable name

'IPSR' was used into the code while it was reading 'XPSR'.
pyOCD/gdbserver: Fixed index out of range in lastSignal()

It happens when the debugger had been stopped in a interrupt
handler.
Merge pull request #15 from oliviermartin/gdbserver-ipsr-fixes

Fixes for GDB Server