Currently, tests of the same kind are grouped together across all types
into a single function. This makes it difficult to understand exactly
what failed in CI.
Change test macros to create separate functions for separate types so
failures are more fine grained.
This adds the truncdfsf2 intrinsic and a corresponding fuzz test case. The
implementation of trunc is generic to make it easy to add truncdfhs2 and
truncsfhf2 if rust ever gets `f16` support.
