[RISCV] Properly evaluate fixup_riscv_pcrel_lo12

This is a update to D43157 to correctly handle fixup_riscv_pcrel_lo12.

Notable changes:

Rebased onto trunk
Handle and test S-type
Test case pcrel-hilo.s is merged into relocations.s

D43157 description:
VK_RISCV_PCREL_LO has to be handled specially. The MCExpr inside is
actually the location of an auipc instruction with a VK_RISCV_PCREL_HI fixup
pointing to the real target.

Differential Revision: https://reviews.llvm.org/D54029
Patch by Chih-Mao Chen and Michael Spencer.

llvm-svn: 349764