Store the "uplevel" target in DWARF assembler

This changes the DWARF assembler to store the desired "uplevel" frame
in Dwarf::assemble.  This frame is then used in the relevant "uplevel"
invocations in the assembler.

This is primarily useful for the next patch, but it also seems
somewhat cleaner in general to me, as the implementation can now be
refactored without worrying too much about introducing new stack
frames.