[GDB/Linux] Avoid stale errno

The fix that went into GDBserver is also needed on the GDB side.

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 may well clobber errno when we get to
evaluate the third argument to fprintf_unfiltered.

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

	* linux-nat.c (kill_callback): Save errno and work with saved
	copy.