gdbserver: dump 'xx...x' in collect_register_as_string for unavailable register

gdbserver: dump 'xx...x' in collect_register_as_string for unavailable register

Fix 'collect_register_as_string' so that unavailable registers are
dumped as 'xx...x' instead of arbitrary values, in particular when
reporting expedited registers in a resume reply packet.  This change
gives the opportunity that we can reuse 'collect_register_as_string'
in 'registers_to_string' for additional code simplification.

Reviewed-By: Luis Machado <luis.machado@arm.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>