libzpool: add workaround for MIPS-specific llvm bug compiling arc.c

This works around an assertion in llvm's shrink wrapping analysis pass,
when compiling arc.c for mips and mips64:

Assertion failed: (PhysReg.isPhysical() && "Unallocated register?!"), function useOrDefCSROrFI, file /usr/src/contrib/llvm-project/llvm/lib/CodeGen/ShrinkWrap.cpp, line 341.

The bug will also be submitted upstream.

Direct commit to stable/13, since MIPS support was removed in 14.0.