section-select: Completely rebuild matches

The check_relocs callback (and others) might have created new
section behind our back and some of them (e.g. on powerpc the
"linker stubs" .got) need to come in front of all others, despite
being created late (a symptom would be "TOC opt*" failing on powerpc).

This resets all section matches before updating for newly created
sections (i.e. completely rebuilds the matches).