linux-nat: Add function lwp_is_stepping

linux-nat: Add function lwp_is_stepping

Add the function lwp_is_stepping which indicates whether the given LWP
is currently single-stepping.  This is a common interface, usable from
native GDB as well as from gdbserver.

gdb/gdbserver/ChangeLog:

	* linux-low.c (lwp_is_stepping): New function.

gdb/ChangeLog:

	* nat/linux-nat.h (lwp_is_stepping): New declaration.
	* linux-nat.c (lwp_is_stepping): New function.