Merge FindSSE and FindAVX cmake files

[kunaltyagi]

A cleaner solution might be to unify pcl_find_sse.cmake and pcl_find_avx.cmake, and have a single variable (e.g. ARCH_FLAGS) that contains sse, avx, and march flags, instead of SSE_FLAGS and AVX_FLAGS. But I would say the current state of this PR still works, and in the interest of getting this merged soon I would propose to not do any major changes here.
I think the cmake options PCL_ENABLE_SSE, PCL_ENABLE_AVX, and PCL_ENABLE_MARCHNATIVE give good control over which flags are added.

Originally posted by @mvieth in #4698 (comment)