This patch is essentially Bernd's 005-scanfirst patch, updated for
the introduction of the dwarf2cfi.c file.
We introduce NOTE_INSN_CFI and NOTE_INSN_CFI_LABEL to hold the dwarf2
info during the bulk of final. The actual construction of these notes
still happens during final, right at the very beginning of the pass,
via the dwarf2out_frame_debug_init hook.