Support ALTIVEC vector instruction on IBM POWER.

[kif]

This issue the the continuation of:
Blosc/c-blosc#267

As stated there, SSE2 instructions from Intel can be transparently mapped into ALTIVEC starting with GCC-8. I made a quick demonstrator which is 40 to 50% faster (bitshuffle-lz4) compared to the master on a power9. The roadmap is the following:

• Add the -DNO_WARN_X86_INTRINSICS compiler flag
• Check the compiler version and processor version
• Update the tests to validate this path like all the others