RISC-V: reduce redundancy in load/store macro insn handling

RISC-V: reduce redundancy in load/store macro insn handling

Within the groups L{B,BU,H,HU,W,WU,D}, S{B,H,W,D}, FL{H,W,D,Q}, and
FS{H,W,D,Q} the sole difference between the handling is the insn
mnemonic passed to the common handling functions. The intended mnemonic,
however, can easily be retrieved. Furthermore leverags that Sx and FSx
are then handled identically, too, and hence their cases can also be
folded.