[RISCV] Implement getExprForFDESymbol to ensure RISCV_32_PCREL is used for the FDE location

Follow binutils in using RISCV_32_PCREL for the FDE initial location. As
explained in the relevant binutils commit
<https://github.com/riscv/riscv-binutils-gdb/commit/a6cbf936e3dce68114d28cdf60d510a3f78a6d40>,
the ADD/SUB pair of relocations is problematic in the presence of linker
relaxation.

This patch has the same end goal as D64715 but includes test changes and
avoids adding a new global VariantKind to MCExpr.h (preferring
RISCVMCExpr VKs like the rest of the RISC-V backend).

Differential Revision: https://reviews.llvm.org/D66419

llvm-svn: 369375