[llvm] [RISCV][GISel] Add FP calling convention support (PR #69138)

[Craig Topper via llvm-commits]

topperc wrote:

> Do we also want to add test cases for using `float` as incoming/outgoing type? since it uses different register classes other than `$x1-$x31`.

Like `callee_float_in_fpr` in calling-conv-ilp32f-ilp32d-common.ll?

https://github.com/llvm/llvm-project/pull/69138