Complete the transition of -split-objs into a dynamic flag (looks like I
half-finished it in the last commit).

Also: complete the transition of -tmpdir into a dynamic flag, which
involves some rearrangement of code from SysTools into DynFlags.

Someday, initSysTools should move wholesale into initDynFlags, because
most of the state that it initialises is now part of the DynFlags
structure, and the rest could be moved in easily.