Introduce linux_proc_pid_to_exec_file

Introduce linux_proc_pid_to_exec_file

This commit introduces a new function linux_proc_pid_to_exec_file
that shared Linux code can use to discover the filename of the
executable that was run to create a process on the system.

gdb/ChangeLog:

	* nat/linux-procfs.h (linux_proc_pid_to_exec_file):
	New declaration.
	* nat/linux-procfs.c (linux_proc_pid_to_exec_file):
	New function, factored out from...
	* linux-nat.c (linux_child_pid_to_exec_file): ...here.