Workaround GDBserver register cache management

Workaround GDBserver register cache management

Given X and C registers have an overlap, if we write to an X register, we need
to fetch the C registers again so we get updated values (side effects).  The
same happens when we attempt to write to the lower half of C registers, we need
to fetch the X registers again so we get updated values.

This patch forces the register cache to get updated values after a register
store request.