Fix #1135: fast Floats + on-demand compliance for Floats.

Floats now use the built-in Math.fround for its operations if the
built-in exists (in which case we hope the VM will optimize it).

Additionally, with a semantics config:
  set scalaJSSemantics ~= { _.withStrictFloats(true) }
the linker will produce strict floats, which always behave as on
the JVM (32-bit precision). On VMs that do not support
Math.fround, we use one of two polyfills: either one based on
Float32Array if that is available, or a slow one that does not
require anything fancy.