[elf2] Sort output sections.

Sort by:
ALLOC
ALLOC && NOBITS
ALLOC & EXEC
ALLOC & EXEC && NOBITS
ALLOC & WRITE
ALLOC & WRITE && NOBITS
<nothing> (ignoring NOBITS)

The dynamic section is finalized early because it adds strings to the dynamic string table, which comes before the dynamic table.

llvm-svn: 248845