makesyscalls.lua: generate all files in /tmp, write into place at the end

[kevans]

Summary

This makes makesyscalls.lua more parallel-friendly, or at least not as hostile to the idea. We get into situations where we're running parallel if we end up with MAKE_JOBS>1 entering any of the sysent targets, since each output file is recognized a distinct build step that needs to be executed.

A later commit should restructure the individual Makefiles to ensure makesyscalls.lua is serialized per-sysent rather than executing it once per generated file, but with this one we at least have a potentially much smaller window to interlace output.

Reported by: jhb

[brooks]

This seems fine and won't be hard to merge with our local changes.