[JITLink][RISCV] Only generate PLT entries for external symbols

[JITLink][RISCV] Only generate PLT entries for external symbols

R_RISCV_CALL has been deprecated. [1] Both GCC and LLVM seem to not
generate it anymore and always use R_RISCV_CALL_PLT (even for calls that
do not need a PLT entry). Generating PLT entries based on relocation
type is not recommended and a better heuristic is to only generate them
when the target symbol is preemptable [2]. This patch implements this by
only generating PLT entries for undefined symbols.

[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/340
[2] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/98

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D149525