revert mips (32-bit, o32) syscall asm clean-up due to regressions

revert mips (32-bit, o32) syscall asm clean-up due to regressions

exactly revert commit 604f8d3d8b08ee4f548de193050ef93a7753c2e0 which
was wrong; it caused a major regression on Linux versions prior to
2.6.36. old kernels did not properly preserve r2 across syscall
restart, and instead restarted with the instruction right before
syscall, imposing a contract that the previous instruction must load
r2 from an immediate or a register (or memory) not clobbered by the
syscall.