fix(Build): Disable Parallel Execution When Creating common_riscv.ld for RISCV_LOAD=1

[Jake-Carter]

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Description

As of #653 some build errors starting showing up in the auto-tester, but just for the AI85 Aud01_RevA BSP. (ex here, here)

This turned out to be an issue with parallel builds. RV_ARM_Loader for the AI85/AI87 failed for other boards with make -r -j, but randomly. The rule for creating riscv_common.ld seems to have an incomplete dependency tree. I tried several solutions to make the dependency tree more explicit, but nothing seemed to work. The file operations for parsing the .map file for the RISC-V boot address always had some chance of modifying files that were being worked on by other parallel threads.

TLDR: As a solution, I've disabled parallel builds for the step where RISCV_LOAD=1 creates riscv_common.ld. Impact on build speed is minimal, and this seems to work.

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