[RISCV] Pass -u to linker correctly.

`-u` is a linker option used to pretend a symbol is undefined,
this option are common used for forcing archive member extraction.

This option should pass to `ld`, and many other toolchain in Clang
like `tools::gnutools` has pass that too.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D105091