Asm versions of netstack memcpy() and lwip_standard_chksum()

For tests such as TCPEchoServer
(http://mbed.org/users/emilmont/notebook/networking-libraries-benchmark/)
this change showed a 28% improvement (14Mbps to 18Mbps) when the echo
test was modified to instead use 1K data buffers.

I targetted these two functions based on manual profiling samples which
showed that a great deal of time was being spent in these two functions
when the network stack was being slammed with UDP packets.
Merge pull request #51 from adamgreen/netMorePerformanceWork

Asm versions of netstack memcpy() and lwip_standard_chksum()

[Note] I'm generally a bit reluctant when including optimizations like this (from an architectural standpoint), because they tend to be a bit too specific (for example, this one works only with lwIP+GCC+Cortex-M3 or M4), but for now it looks as this is the right place for them, although the optimized memcpy should ideally be in libc (or even better replaced with a DMA transfer in this particular case). But this will be both a nice optimization and a reminder of what we need to implement/change in the future.