Need .tds for libm and runtime libcall API for ~all LLVM targets

[workingjubilee]

This was raised as a concern in #111269 and I am opening an issue to make sure it is not lost as a desired followup.

Basically, there are two contending needs for a list of libm functions:

• Frontends want to know what builtin functions LLVM will need to call at runtime
• LLVM wants to keep track of actual calls to libm functions for LTO purposes

Frontends see RuntimeLibcalls.def as suiting the former, but LLVM has lately grown some... appropriations of the .def as the latter. However, RuntimeLibcalls.def is inadequate for the latter, even if it has proven a convenient start. It could use being promoted to a proper TableGen file that can better handle the obviously parameterized cases here.

It might need to be multiple files, given the distinct usages, but hopefully we can avoid complete duplication.