Add toggle to disable SIMD speed test added in 2a7127fea6
Add toggle to disable SIMD speed test added in 2a7127fea6

Failed to consider need for ISA flags (none required on x86-64 or
aarch64 which are my primary test targets) but handling this correctly
requires splitting up speed.cpp which requires a lot more refactoring
work than I want to take on. Anyway the additional complexity just for
this microbenchmark is not worth it so hide it behind a macro check.

Also add an ECC scalar perf test.

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