This adds runtime support to use Zbb REV8 for 32- and 64-bit byte-wise
swaps. The result is about five times slower than if targetting Zbb
statically, but still a lot faster than the default bespoke C code or a
call to GCC run-time functions.

For 16-bit swap, this is however unsurprisingly a lot worse, and so this
sticks to the baseline. In fact, even using REV8 statically does not
seem to be beneficial in that case.

         Zbb static    Zbb dynamic   I baseline
bswap16:  0.668184765   3.340764069   0.668029012
bswap32:  0.668174014   3.340763319   9.353855435
bswap64:  0.668221765   3.340496313  14.698672283
(seconds for 1 billion iterations on a SiFive-U74 core)