Make linux_stop_lwp be a shared function

Make linux_stop_lwp be a shared function

Both GDB and gdbserver had linux_stop_lwp functions with identical
declarations.  This commit moves these to nat/linux-nat.h to allow
shared code to use the function.

gdb/ChangeLog:

	* linux-nat.h (linux_stop_lwp): Move declaration to...
	* nat/linux-nat.h (linux_stop_lwp): New declaration.

gdb/gdbserver/ChangeLog:

	* linux-low.h (linux_stop_lwp): Remove declaration.