autotools: intel cpu features detection

Rename SIMD to INTEL for clarity.

Instead of agregating all flags in INTEL_FLAGS, create individual flags
for each feature (INTEL_SSE2_FLAGS etc.) for finer control in the
makefiles.

Signed-off-by: Loic Dachary <loic@dachary.org>
arch: add SSSE3 and PCLMUL intel features

And add a note about valgrind forcing a fake cpuid.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: SSE optimized jerasure plugins

The jerasure plugin is compiled with three sets of flags:

* jerasure_generic with no SSE optimization
* jerasure_sse3 with SSE2 and SSSE3 optimizations
* jerasure_sse4 with SSE2, SSSE3, SSE4 and PCLMUL optimizations

The jerasure plugin loads the appropriate plugin depending on the CPU
features detected at runtime.

Signed-off-by: Loic Dachary <loic@dachary.org>