[RISCV] Register null target streamer for RISC-V

Summary:
This fixes two llc crashes with the following tests when RISC-V is the default
target.

LLVM :: DebugInfo/Generic/global.ll
LLVM :: DebugInfo/Generic/inlined-strings.ll

Reviewers: HsiangKai

Reviewed By: HsiangKai

Subscribers: hiraditya, asb, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, lenary, s.egerton, sameer.abuasal, apazos, luismarques, evandro, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80352