Test little and big endian architectures in CI

[juntyr]

We test little endian architectures in the numpy CI using PPC64le QEMU emulation. It's slow but it at least lets you do a real end-to-end test.

Originally posted by @ngoldbaum in #118 (comment)

[juntyr]

@SwayamInSync Maybe you have an idea for how to set this up? Once the infrastructure is there, I could then implement the signbit ufunc with its binary implementation (and follow that up with nextafter).

[SwayamInSync]

"PPC64le" is also little-endian (we are compatible & well tested on little endian), I can try emulating the Big-Endian variants like PPC64 and s390x with QEMU

[ngoldbaum]

Ah yeah, sorry, got it backwards!