RISC-V: Don't segfault for two regs in auipc or lui.

RISC-V: Don't segfault for two regs in auipc or lui.

	gas/
	PR gas/23954
	* config/tc-riscv.c (my_getSmallExpression): Expand comment for
	register support.  Set expr_end if parse a register.
	(riscv_ip) <'u'>: Break if imm_expr is not a symbol or constant.
	* testsuite/gas/riscv/auipc-parsing.d: New.
	* testsuite/gas/riscv/auipc-parsing.l: New.
	* testsuite/gas/riscv/auipc-parsing.s: New.