gdb: Avoid using W_STOPCODE(0) as this is ambiguous on MIPS

The MIPS target supports 127 signals, and this can create an ambiguity
in process wait statuses.  A status value of 0x007f could potentially
indicate a process that has exited with signal 127, or a process that
has stopped with signal 0.

In uClibc-ng the interpretation of 0x007f is that the process has
exited with signal 127 rather than stopped with signal 0, and so,
WIFSTOPPED (W_STOPCODE (0)) will be false rather than true as it would
be on most other platforms.

Given that it's pretty easy to avoid using W_STOPCODE (0), lets do that.

gdb/ChangeLog:

	* linux-nat.c (linux_nat_target::follow_fork): Avoid using
	'W_STOPCODE (0)' as this could be ambiguous.