gdbserver: make linux target op 'cannot_store_register' a predicate function

gdbserver: make linux target op 'cannot_store_register' a predicate function

The comment for the linux target op 'cannot_store_register' states the
following:

  /* Returns 0 if we can store the register, 1 if we can not
     store the register, and 2 if failure to store the register
     is acceptable.  */

There is only one low target, linux-ppc-low, that potentially returns
2.  There are two places that call the 'cannot_store_register' target
op in linux-low.cc.  None of these locations distinguish a '2' from a
'1'.  Hence, to simplify the definition, make the function a predicate
that returns either 0 or 1.  This is also consistent with the
companion function, 'cannot_fetch_register'.

gdbserver/ChangeLog:
2020-04-02  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* linux-low.h (struct linux_target_ops): Update the comment for
	'cannot_store_register' to return 0 or 1.
	* linux-ppc-low.cc (ppc_cannot_store_register): Return 1 instead
	of 2.