Support dead-stripping in ELF objects

[Robinson, Paul]

Okay, just to restate the specification all in one place.

We define three new ELF section-header flags:

* SHF_ATOMIZED
The symbols defined for this section (other than the STT_SECTION symbol)
have the following properties.
- The range [st_value, st_value + st_size) for each symbol does not overlap
  the range for any other symbol.
- For all symbols, st_value + st_size <= sh_size (the end of the section).
- All relocations targeting this section shall resolve to values within the
  range for some symbol.

The range for each symbol is called an "atom."  There are no relocations
addressing any gaps between atoms.

Note in particular this does not disallow using .section+N relocations,
as long as the target address is in the range of some atom.

(Michael, is it really required to have a symbol with st_value = 0, if
we have all the other requirements?  Typically we would, but strictly
speaking a "gap" at the start of the section should not be a problem?)

* SHF_SUBSECTIONS_VIA_SYMBOLS
SHF_ATOMIZED must be set if SHF_SUBSECTIONS_VIA_SYMBOLS is set.
In addition, the following properties hold for the entire object file.
- No relocation uses the STT_SECTION symbol for this section.
- If any relocation uses a symbol defined in this section, the addend
  must be less than st_size for that symbol.
- The range [st_value, st_value + st_size) of any symbol in the section
  may be moved to a different relative location.

Each "atom" in this section is also called a "subsection."  Any relocation
using the symbol for subsection must resolve to an address within the
subsection.  There are no location dependencies between subsections, other
than those expressed by relocations.

* SHF_DEADSTRIP
This entire section may be omitted from the output file, if it is dead
(i.e., there are no references to any symbol defined in the section).
If SHF_SUBSECTIONS_VIA_SYMBOLS is also set, individual subsections
may be omitted from the output file, if they are dead.


We define one new ELF symbol flag:

* STF_NO_DEADSTRIP
The linker may not remove this symbol from the output file.
This symbol flag takes precedence over the SHF_DEADSTRIP section flag.
If the symbol defines a subsection, the subsection must be considered live.
If the symbol does not define a subsection, the symbol's entire section
must be considered live.


How's that?
--paulr