Detect broken FP toolchains at runtime
When we have a broken floating-point implementation, stuff can get weird and people can see weird bugs. See #1194 (moved), #6767 (moved), #6697 (moved) .
I think we could save ourselves some headaches and brain cells if it's possible to write a simple spot-check for the bad behavior described in #1194 (moved). I'm not sure how hard that would be though.