Asm versions of netstack memcpy() and lwip_standard_chksum()

I just targeted the GCC_ARM toolchain with these changes to minimize
risk.  For the same reason, I only targeted this memcpy() to be used by
the network stack.

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.