Handle multilib config correctly

`--build-arch-abi` may give more than one arch/abi pair if multilib
enabled, but we don't handle that well before...

Before the patch:
```bash
$ scripts/generate_target_board --sim-name riscv-sim --cmodel medlow \
  --build-arch-abi "rv32imac-ilp32 rv32imafdc-ilp32d rv64imac-lp64 rv64imafdc-lp64d" \
  --extra-test-arch-abi-flags-list ""
riscv-sim/-march=rv32imac/-mabi=ilp32 rv32imafdc/-mcmodel=medlow
```

After the patch:
```bash
$ scripts/generate_target_board --sim-name riscv-sim --cmodel medlow \
  --build-arch-abi "rv32imac-ilp32 rv32imafdc-ilp32d rv64imac-lp64 rv64imafdc-lp64d" \
  --extra-test-arch-abi-flags-list ""
riscv-sim/-march=rv32imac/-mabi=ilp32/-mcmodel=medlow riscv-sim/-march=rv32imafdc/-mabi=ilp32d/-mcmodel=medlow riscv-sim/-march=rv64imac/-mabi=lp64/-mcmodel=medlow riscv-sim/-march=rv64imafdc/-mabi=lp64d/-mcmodel=medlow
```