[Bug middle-end/50481] builtin to reverse the bit order

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

--- Comment #6 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
As noted in the RISC-V BoF today, this would also be useful for the RISC-V
bit-manipulation extension.