Introduce a new class for the TargetLowering usage. This tracks the
subtarget specific lowering decisions for which libcall to use.
RuntimeLibcallsInfo is a module level property, which may have multiple
implementations of a particular libcall available. This attempts to be
a minimum boilerplate patch to introduce the new concept.
In the future we should have a tablegen way of selecting which
implementations should be used for a subtarget. Currently we
do have some conflicting implementations added, it just happens
to work out that the default cases to prefer is alphabetically
first (plus some of these still are using manual overrides
in TargetLowering constructors).
Eventually this should be generated by tablegen for all functions.
For now add a manually implementation for sincos_stret, which I
have an immediate use for. This will allow pulling repeated code
across targets into shared call sequence code.
Also add sqrt just to make sure we can handle adding return attributes
on the declaration.
This will be useful for testing the set of calls for different systems,
and eventually the product of context specific modifiers applied. In
the future we should also know the type signatures, and be able to
emit the correct one.
