[RISCV] Implement floating point assembler pseudo instructions

Adds the assembler aliases for the floating point instructions
which can be mapped to a single canonical instruction. The missing
pseudo instructions (flw, fld, fsw, fsd) are marked as TODO. Other
things, like for example PCREL_LO, have to be implemented first.

This patch builds upon D40902.

Differential Revision: https://reviews.llvm.org/D41071

Patch by Mario Werner.

llvm-svn: 320569