[RISCV] Support mutilib in baremetal environment

1. Currently only support the set of multilibs same to riscv-gnu-toolchain.
2. Fix testcase typo causes fail on Windows.
3. Fix testcases to set empty sysroot.

Reviewers: espindola, asb, kito-cheng, lenary

Reviewed By: lenary

Differential Revision: https://reviews.llvm.org/D67508