[clang][RISCV][test] Add test cases for empty structs and the FP calling conventions

As reported in https://github.com/llvm/llvm-project/issues/58929, Clang
currently differs from GCC in the handling of empty structs. This commit
adds some test coverage for the handling of such structs.

A follow-up patch implements a fix to match g++.

Differential Revision: https://reviews.llvm.org/D142326