[RISCV][test] Add test coverage for llvm.frexp.*.* intrinsics

The test file is copied from X86 (which is also mostly shared with Arm,
PowerPC) rather than integrated into float-intrinsics.ll and
double-intrinsics.ll.

There's currently a compiler crash for the soft float cases (expect this
is the issue in <https://github.com/llvm/llvm-project/issues/63661>)
which will be a addressed with a follow-on patch posted for review.