gdb/nat: Factor linux_proc_get_stat_field out of linux_common_core_of_thread

The new function will be used in a subsequent patch to read a different
stat field.

The new code is believed to be equivalent to the old code, so there
should be no change in GDB behaviour.  The only material change was to
use std::string and string_printf rather than a fixed char array to
build the path to the stat file.

Also, take the opportunity to move the function's documentation comment to
the header file, to conform with GDB practice.

Reviewed-By: Luis Machado <luis.machado@arm.com>
Approved-By: Pedro Alves <pedro@palves.net>