build: Cache checks for SIMD implementations

We want to cache the results, but more importantly we want to have a
named variable that can be overridden when cross-compiling for known
architectures.
simd4f: Fix scalar macros

Some typos.
simd4f: Fix scalar cmp_neq()

DeMorgan would be very, very cross with me.
simd4f: Fix get() operator

Typo.
tests: Use fuzzy comparisons

The scalar implementation of SIMD4f introduces a lot more fuzziness in
the results of divisions than the equivalent optimized paths.