[LLVM][RUNTIME] Fix RISC-V CodeModel propagation to ORCJIT runtime executor

[cbalint13]

This PR fixes ORCJIT link failures for RISC-V targets by properly setting the code model on the executor.

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Initial bug report is here: https://discuss.tvm.apache.org/t/run-tvm-software-stack-on-risc-v/17683

Rationale

• RISC-V recommended and required code model was set already here, but was ignored:

  tvm/src/target/llvm/llvm_instance.cc

  Lines 276 to 280 in e468426

  	// RISCV code model
  	auto arch = llvm::Triple(triple_).getArch();
  	if (arch == llvm::Triple::riscv32 || arch == llvm::Triple::riscv64) {
  	code_model_ = llvm::CodeModel::Medium;
  	}

• RISC-V targets will work only with the newer ORCJIT engine as described in PR#15964
• This PR adds missing code model settings for the ORCJIT engine.

In case of RISC-V, by not setting the code model to explicit Medium may give ralocation R_RISCV_HI20 errors.

Cc: @Lunderberg , @ekalda , @lhutton1 , @quic-sanirudh , @tqchen
Cc: @katebern-grovety , @PhilippvK

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LATER UPDATE (due to unexpected i386 failure):

• This PR propagates explicit code/reloc model only for RISC-V arches.
• Just as before, by default will leave the ORCJIT to decide the best code/reloc
• Same JIT setup, and RISCV case handling can be seen in the Halide ORCJIT

[quic-sanirudh]

LGTM, Thanks.