RISC-V: Fix build failures for -Werror=sign-compare.

elfnn-riscv.c: In function ‘riscv_relax_resolve_delete_relocs’:
elfnn-riscv.c:4256:30: error: operand of ‘?:’ changes signedness from ‘int’ to ‘unsigned int’ due to unsignedness of other operand [-Werror=sign-compare]

So make the operands unsigned could resolve problem.

bfd/
    * elfnn-riscv.c (riscv_relax_resolve_delete_relocs): Fixed build
    failures for -Werror=sign-compare.