Adds conditional byteswapping intrinsics

These intrinsics are synthesized, so maybe they should be in another
file. But since they are just a single line of code each, based on the
bswap intrinsics and aren't really intended for public consumption (they should be exposed as a
single function / trait) I thought they would fit here.
auto merge of #7194 : jensnockert/rust/endian, r=cmr

They simply byte-swap an integer to a specific endian, like the hton* functions in C.

These intrinsics are synthesized, so maybe they should be in another file. But since they are just a single line of code each, based on the bswap intrinsics and aren't really intended for public consumption I thought they would fit in the intrinsics file.

The next step working on this could be to expose a trait / generic function for byteswapping.