[GDBserver] Avoid stale errno

[GDBserver] Avoid stale errno

Although most compilers follow right-to-left evaluation order, the
order of evaluation of a function call's arguments is really
unspecified.  target_pid_to_str or ptid_of may well clobber errno when
we get to evaluate the third argument to debug_printf.

gdb/gdbserver/
2014-07-15  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_kill_one_lwp): Save errno and work with saved
	copy.