Support for non-x86-64 SBCL

[Partmedia]

Because I happen to want to run a grpc server written in Common Lisp on GNU/Linux aarch64 (arm64), I'm interested in what it would take to upstream support for this. Most notably, aarch64 is (by default) big endian.

Based on some cursory looking, it looks like only define-fixed-width-encoder needs to be implemented?

For implementing this, would it be acceptable to bring in a whole or parts of a library like swap-bytes? This particular library is available under the MIT license. The strategy could be then to:

• Add htonl/htonq equivalents that convert integers to protobuf wire format (little endian)
• Wrap fixed with integer accessors (e.g. sb-sys:sap-ref-N) to a call to those equivalents

On little-endian machines the conversion functions would become no-ops, and on big-endian machines it would call the internal swap-bytes implementation.

This library currently does not implement a native byte-swap routine for arm64 and would resort to a portable fallback, but a faster implementation using arm64 intrinsics could be upstreamed.

[mdbergmann]

Not sure. But it works here on aarch64 on Mac. Is this different from aarch64 Linux?