[RISCV] Mark unit tests as "requires: riscv-registered-target"

Summary: Some of these tests break if the RISCV backend has not been built.

Reviewers: asb, apazos, sabuasal

Reviewed By: sabuasal

Subscribers: rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, cfe-commits

Differential Revision: https://reviews.llvm.org/D54816

llvm-svn: 347688