Map registers to remote numbers when encoding an ax_reg or ax_reg_mask operation
When encoding the agent expression operation ax_reg or ax_reg_mask, the
register number used is internal to GDB. However GDBServer expects a tdesc
based number.
This usually does not cause a problem since at the moment, for raw
registers GDBServer R trace action ignores the register mask and just
collects all registers.
It can be a problem, however with pseudo registers on some platforms if the
tdesc number doesn't match the GDB internal register number.
This is the case with ARM, the upcoming ARM tracepoint support, fails
these test cases without this patch: