linux-user: convert /proc/net/route when endianess differs

This patch allows to have IP addresses in correct order
in the case of "netstat -nr" when the endianess of the
guest differs from one of the host.

For instance, an m68k guest on an x86_64 host:

WITHOUT this patch:

$ netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         1.3.0.10        0.0.0.0         UG        0 0          0 eth0
0.3.0.10        0.0.0.0         0.255.255.255   U         0 0          0 eth0
$ cat /proc/net/route
Iface	Destination	Gateway 	Flags	RefCnt	Use	Metric	Mask	MTU	Window	IRTT

eth0	00000000	0103000A	0003	0	0	0	000000000	0	0
eth0	0003000A	00000000	0001	0	0	0	00FFFFFF0	0	0

WITH this patch:

$ netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         10.0.3.1        0.0.0.0         UG        0 0          0 eth0
10.0.3.0        0.0.0.0         255.255.255.0   U         0 0          0 eth0
$ cat /proc/net/route
Iface	Destination	Gateway 	Flags	RefCnt	Use	Metric	Mask	MTU	Window	IRTT
eth0	00000000	0a000301	0003	0	0	0	000000000	0	0
eth0	0a000300	00000000	0001	0	0	0	ffffff000	0	0

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>