[clang][RISCV][test] Add test coverage for _Float16 ABI lowering

By the psABI, any test case where a FPR would be used for a float, it
should also be used if you replaced that float with a _Float16. This
doesn't hold true in current Clang for the special cases in the FP
calling convention involving structs. This patch doesn't attempt to fix
that, simply to add coverage. D145074 contains the fix.

Differential Revision: https://reviews.llvm.org/D145070