Add RISC-V filter support

[Larhzu]

This requires liblzma >= 5.6.0. The LZMA2 options are set with the assumption that the RISC-V C extension is in use.

I have submitted the RISC-V filter to Linux. It's in the -mm mm-nonmm-unstable branch at the moment.

The ARM64 filter is there too, that is, it didn't get into Linux 6.7 or 6.8. I failed to ensure that it would get included. Sorry.

(Not about this PR but it could be good if the default LZMA2 options could be changed for the old filters too: ARM, PowerPC, and SPARC would use the same as ARM64, and ARM-Thumb the same as RISC-V. IA-64 would need pb=4,lp=4,lc=0 but the IA-64 filter was disabled in Linux 6.7, possibly by accident, when support for the IA-64 arch was removed.)