Rework x86 targets to allow for x64-64 without sse4.1, plus related changes (Issue #100)

-- ensure that all x86 targets gave explicit 32 or 64 suffixes (rather than having x86 imply 64-bit)
-- x86-64 now doesn't assume SSE4.1 (or SSSE3), so is suitable for all x86-64 architectures; added a new x86-64-sse41 to support SSE4.1 enabled x86-64. (Note that we now enable SSSE3 instructions only when SSE4.1 is enabled; previously we assumed all x86 variants were SSSE3-capable, which made it impossible to support all x86-64 architectures)
-- added NaCl targets corresponding to every x86 target
-- revamped the runtime support files as necessary to match the new set of targets
-- added some smarts to tests/simd_op_check.cpp to skip checking ssse3/sse41/sse42 when not part of the target