devel/py-pythran: fix bundled xsimd VSX backend with clang

pythran 0.19.0 bundles xsimd 14.1, whose new VSX backend uses
vector long / vector bool long, which clang rejects on PowerPC.
Backport upstream xsimd commit 8df8da7 (released in 14.3.0).

Fixes science/py-scipy on powerpc64 and powerpc64le.