Currently .dwo sections are usually emitted to .dwo file.
I am working on a single file split case, which is mentioned in
the DWARF5 spec. It says that
"The sections that do not require relocation, however, can be written
to the relocatable object (.o) file but ignored by the
the linker or they can be written to a separate DWARF object (.dwo) file
that need not be accessed by the linker."
Nice way to make linker to ignore them is to set SHF_EXCLUDE flag.
I believe it is not harmful to always set it for .dwo sections.
That is what this patch does.