bitops.h: Implement half-shuffle and half-unshuffle ops

bitops.h: Implement half-shuffle and half-unshuffle ops

A half-shuffle operation takes a word with zeros in the high half:
 0000 0000 0000 0000 ABCD EFGH IJKL MNOP
and spreads the bits out so they are in every other bit of the word:
 0A0B 0C0D 0E0F 0G0H 0I0J 0K0L 0M0N 0O0P
A half-unshuffle performs the reverse operation.

Provide functions in bitops.h which implement these operations
for 32-bit and 64-bit inputs, and add tests for them.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Shannon Zhao <[email protected]>
Tested-by: Shannon Zhao <[email protected]>
Message-id: 1465915112[email protected]