Fix support for i686 32-bit target. (#923)

* Use alternative hashing of cell_member_type values for 32-bit target.
* Correctly match i686 as a "-march=" style target for gcc and clang.
* Correct a signed/unsigned comparison warning in test_algorithms.cpp with -Wall.
* Avoid spurious SIMD unit test failure for exprelr by using expm1 instead of exp - 1 in denominator when computing expected results.