cmd/internal/obj/riscv: rework instruction encoding information

Currently, instruction encoding is a slice of encoding types, which
is indexed by a masked version of the riscv64 opcode. Additional
information about some instructions (for example, if an instruction
has a ternary form and if there is an immediate form for an instruction)
is manually specified in other parts of the assembler code.

Rework the instruction encoding information so that we use a table
driven form, providing additional data for each instruction where
relevant. This means that we can simplify other parts of the code
by simply looking up the instruction data and reusing minimal logic.

Change-Id: I7b3b6c61a4868647edf28bd7dbae2150e043ae00
Cq-Include-Trybots: luci.golang.try:gotip-linux-riscv64
Reviewed-on: https://go-review.googlesource.com/c/go/+/622535
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Meng Zhuo <[email protected]>
Reviewed-by: Mark Ryan <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: 4a6f656c