gdb/infrun: extract out a code piece into 'mark_non_executing_threads' function

gdb/infrun: extract out a code piece into 'mark_non_executing_threads' function

This is a refactoring.  The extracted function is placed deliberately
before 'stop_all_threads' because the function will be re-used there
in a subsequent patch for handling an exit status kind received from
a thread that GDB attempted to stop.

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

	* infrun.c (handle_inferior_event): Extract out a piece of code
	into...
	(mark_non_executing_threads): ...this new function.

Change-Id: I2b088f4a724f4260cb37068264964525cf62a118