Enable (fix?) VSX for ppc64 (big endian)

[barracuda156]

@kif Hi, bringing up the old issue. CMakeLists still require LE build for ppc64: https://github.com/Blosc/c-blosc2/blob/main/CMakeLists.txt#L374-L381
While FreeBSD and Debian (among other platforms) presently support ppc64 (BE). Is it possible to fix VSX implementation for those (on cpus supporting ISA 2.06+)?

Related: #592
#619

[kif]

Sorry, I don't have access to any PPC64BE computer, so I cannot help much for that.

[barracuda156]

Just in case, any modern Power machine is bi-endian (the same hardware which LE was tested on can do BE). Probably even emulated in Qemu. Or remote access from OpenPower or GNU programs.