gdb: protect some 'regcache_read_pc' calls

It possible that a thread whose PC we attempt to read is already dead.
In this case, 'regcache_read_pc' errors out.  This impacts the
"proceed" execution flow, where GDB quits early before having a chance
to check if there exists a pending event.  To remedy, keep going with
a 0 value for the PC if 'regcache_read_pc' fails.  Because the value
of PC before resuming a thread is mostly used for storing and checking
the next time the thread stops, this tolerance is expected to be
harmless for a dead thread/process.

gdb/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* regcache.c (regcache_read_pc_protected): New function
	implementation that returns 0 if the PC cannot read via
	'regcache_read_pc'.
	* infrun.c (proceed): Call 'regcache_read_pc_protected'
	instead of 'regcache_read_pc'.
	(keep_going_pass_signal): Ditto.

gdbsupport/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* common-regcache.h (regcache_read_pc_protected): New function
	declaration.

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