This patch renames values of dsymutil/llvm-dwarfutil options:
--linker apple -> --linker classic
--linker llvm -> --linker parallel
The purpose to rename options is to avoid using vendor names and to
match with library names. It should be safe to rename options at current
stage as they are not seemed widely used(we may not preserve backward
compatibility).
There was a strange and seemingly unncessary empty string optimization
in NonRelocatableStringPool that I had to remove in order to support
empty strings in the line_str string table, without unconditionally
forcing an empty string to be added to every debug_line_str table.