[RISCV] Add rv32i/rv64i command lines to some floating point tests. NFC

This improves our coverage of soft float libcalls lowering.

Remove most of the test cases from rv64i-single-softfloat.ll. They
were duplicated in the test files that now test softflow. Only
a couple test cases for constrained FP remain. Those should be
removed when we start supporting constrained FP.

This is follow up from D113528.