In the builtins library, most functions have a portable C implementation
(e.g. `mulsf3.c`), and platforms might provide an optimized assembler
implementation (e.g. `arm/mulsf3.S`). The cmake script automatically
excludes the C source file corresponding to each assembly source file it
includes. Additionally, each source file name is automatically
translated into a flag that lit tests can query, with a name like
`librt_has_mulsf3`, to indicate that a function is available to be
tested.
In future commits I plan to introduce cases where a single .S file
provides more than one function (so that they can share code easily),
and therefore, must supersede more than one existing source file.
I've introduced the `crt_supersedes` cmake property, which you can set
on a .S file to name a list of .c files that it should supersede. Also,
the `crt_provides` property can be set on any source file to indicate a
list of functions it makes available for testing, in addition to the one
implied by its name.
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