[RISCV] Add InstAlias definitions for fgt.{s|d}, fge.{s|d}

These are produced by GCC and supported by GAS, but not currently contained in 
the pseudoinstruction listing in the RISC-V ISA manual.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335127 91177308-0d34-0410-b5e6-96231b3b80d8