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

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

This commit tests both (assembler and disassembler) fixes of "Better support
for long instructions".

gas/ChangeLog:

	* testsuite/gas/riscv/insn.s: Add testcases such that big number
	handling is required and should be disassembled as long ".byte"
	sequence with correct instruction bits.
	* testsuite/gas/riscv/insn.d: Likewise.
	* testsuite/gas/riscv/insn-na.d: Likewise.
	* testsuite/gas/riscv/insn-dwarf.d: Likewise.