[ELF][RISCV] Add R_RISCV_PC_INDIRECT to isRelExpr()

So that R_RISCV_PCREL_LO12_[IS] are considered as link-time constants in
-pie mode, otherwise there are bogus errors:

    ld.lld: error: can't create dynamic relocation R_RISCV_PCREL_LO12_I against symbol: .L0  in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D63123

llvm-svn: 363064