PR26493 UBSAN: elfnn-riscv.c left shift of negative value

PR26493 UBSAN: elfnn-riscv.c left shift of negative value

include/
	PR 26493
	* opcode/riscv.h (OP_MASK_CSR, OP_MASK_CUSTOM_IMM)
	(OP_MASK_FUNCT7, OP_MASK_RS3): Make unsigned.
bfd/
	PR 26493
	* elfnn-riscv.c (riscv_make_plt_header): Cast PLT_HEADER_SIZE to
	unsigned when using with RISCV_ITYPE.
	(_bfd_riscv_relax_call): Use an unsigned foff.