Add STB_SECONDARY to gABI

["H.J. Lu" <hjl dot tools at gmail dot com>]

Here is the final proposal to add STB_SECONDARY to gABI.
Any comments?

Thanks.

-- 
H.J.
---
We want to provide a relocatable object which can take advantage of all
versions of a supported OS.  For a function, foo, in the C library, we
can use it only if it is available on all versions of the C library or
we provide our own implementation of foo.  With our own foo, the one in
the C library will never be used.  Here is a proposal to add STB_SECONDARY
to gABI to support the secondary definition so that a software vendor
can provide an alternative implementation in case it isn't available
in the C library.

STB_SECONDARY

      Secondary symbols are similar to weak symbols, but their definitions
      have even lower precedence.  The difference between secondary symbols
      and weak symbols are

	1. The link editor must search archive library and extract
	archive members to resolve defined and undefined secondary symbol.
	2.  When the link editor searches a shared object, it must honor
	the global or weak definition in the shared object and ignore the
	secondary one with the same name.
	3. The link editor ignores the secondary definition if there is
	a global, weak or common definition with the same name.  Multiple
	secondary definitions with the same name will not cause an error.
	The first appearance of the secondary definition should be honored
	and the rest are ignored.
	4. The link editor may treat the secondary definition in the
	shared object as a global definition.
	5. Unresolved secondary symbols have a zero value.

      The purpose of this symbol binding is to provide the primary
      definition as a global, weak or common symbol in an archive library
      or a shared object while keeping a secondary definition in a
      relocatable object.  If there is no primary definition, the
      secondary definition will be used.

      When secondary definitions become part of an executable or shared
      object, linker may convert them to global or local definitions.

      At run-time, when resolving a symbol, after seeing a secondary
      definition, the dynamic linker must keep searching until a
      global or weak definition is found.  If a global or weak
      definition is found, it will be used to satisfy the symbol lookup.
      Otherwise, the secondary definition will be used.

      If the dlopen loads a global or weak definition after the program
      has already resolved references to a secondary definition, those
      references remain bound to the secondary definition.  Any
      references resolved after the dlopen, for which the dlopened
      module is included in the module search list, would be resolved
      to the global or weak definition.

STB_SECONDARY is defined as:

#define STB_SECONDARY	3	/* Secondary symbol */