[RISCV] Fix evalutePCRelLo for symbols at the end of a fragment

Summary:
This is analogous to D58943, which correctly finds the corresponding
fixup. However, when linker relaxations are disabled and we evaluate the
fixup, we need to also ensure we use an offset of 0 rather than the size
of the previous fragment.

Reviewers: asb, efriedma, lenary

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71978