Convert words to bytes endianness-aware before sending to GLIP

[imphil]

• GLIP (glip_read/glip_write) operates on bytes.
• When assembling words out of those bytes, it does so using big endian encoding ("All data transfer is big endian. This means for a 2 byte wide FIFO (check the FIFO width with glip_get_fifo_width()) the first byte written will be the MSB in the FIFO, the second byte written will be the LSB." -- http://www.glip.io/group__communication.html#ga7b3737ebb2d1f601f0c987cf50f0c43a)
• libopensocdebug uses memcpy() to convert uint16 -> uint8_t[2]. On (little endian) x86 systems, this results in byte[0] = LSB, byte[1] = MSB.
• After being transmitted through GLIP, the fifo_out_data[15:0] signal contains fifo_out_data[15:8] = LSB, fifo_out_data[7:0] = MSB.
• Since this is not what all other parts of the OSD spec (or any hardware developer) expect, the byte order is swapped again in HIM (https://github.com/opensocdebug/hardware/blob/master/modules/him/common/osd_him.sv#L24)

This swapping in HIM is a workaround which only works on little endian host systems (i.e. libopensocdebug won't work on power pc).

To solve this issue, we should

• not use memcpy() inside libopensocdebug, but to use a endianness-aware copy function instead
• Remove the byte swapping in HIM
• Clearly spell out this fact in the documentation.