RISC-V: Better support for long instructions (assembler)

Commit bb996692bd96 ("RISC-V/gas: allow generating up to 176-bit
instructions with .insn") tried to start supporting long instructions but
it was insufficient.

1.  It heavily depended on the bignum internals (radix of 2^16),
2.  It generates "value conflicts with instruction length" even if a big
    number instruction encoding does not exceed its expected length and
3.  Because long opcode was handled separately (from struct riscv_cl_insn),
    some information like DWARF line number correspondence was missing.

To resolve these problems, this commit:

1.  Handles bignum (and its encodings) precisely and
2.  Incorporates long opcode handling into regular instruction handling.

This commit will be tested on the separate commit.

gas/ChangeLog:

	* config/tc-riscv.c (struct riscv_cl_insn): Add long opcode field.
	(create_insn) Clear long opcode marker.
	(install_insn) Install longer opcode as well.
	(s_riscv_insn) Likewise.
	(riscv_ip_hardcode): Make big number handling stricter. Length and
	the value conflicts only if the bignum size exceeds the expected
	maximum length.