support riscv64 builds

[threexc]

This makes the changes necessary to support riscv64 builds in pyav-ffmpeg. The work here is being done with the goal of supporting video_reader-rs using pyav-ffmpeg in its CI, which in turn is of interest to the RISE Project.

Note that there's a patch labeled revertme which reduces the test matrix down to just riscv64 for testing purposes.

I started by testing this on my fork: threexc#1

What I found there is that the manylinux build succeeds once cuda and amf are also disabled for riscv64, but musllinux is a different story. I looked to see what version of musl the musllinux_1_2 image includes:

tgamblin@megalith ~ $ docker run --rm --platform linux/riscv64 quay.io/pypa/musllinux_1_2_riscv64:2026.03.01-1 sh -c "apk info musl"
WARNING: opening from cache https://dl-cdn.alpinelinux.org/alpine/v3.22/main: No such file or directory
WARNING: opening from cache https://dl-cdn.alpinelinux.org/alpine/v3.22/community: No such file or directory
musl-1.2.5-r10 description:
the musl c library (libc) implementation

musl-1.2.5-r10 webpage:
https://musl.libc.org/

musl-1.2.5-r10 installed size:
598 KiB

Seems that the missing header (stdbit.h) is a C23 addition

I guess we'd need to wait for musl to support it, and then for a new musllinux_1_2 image to be built?

Alternatively, it looks like there's a compatibility layer for it in ffmpeg, which we could try to use.

[WyattBlue]

x265 and cmake patches should be in a separate PR since it benefits all builds.