ELF Tool Chain Code

Remove the FDE's associated with discarded COMDAT sections. After this
change, the linker can now generate correct and smaller .eh_frame
sections for C++ programs.

Revamped SHT_GROUP (section group) support:
* Process SHT_GROUP sections before resolving symbols. Symbol index
can be changed to undefined and symbols can be discarded when
duplicate sections are discarded during SHT_GROUP section processing.
Symbol index change and symbol removal is better to happen before
symbol resolving.
* No need to keep track of section groups in each input object.
Sections are discarded immediately when duplicates are found.

Add support for SHT_GROUP (section group) sections. SHT_GROUP sections
are treated especially and should not appear in the output executable
or shared library.

Use a flag to indicate whether an input section need to apply
relocations.

Use fixed maximum number of internal section to simplify the code and
to avoid bugs like refering to invalid internal section pointers after
the internal section array buffer is reallocated.

Implement support for creating ".eh_frame_hdr" output section.

Implement function "ld_reloc_gc_sections" that performs a breadth-first
search for sections referenced by relocations associated with "used"
sections, i.e, to find sections that should not be garbage-collected.
The initial set of "used" sections consists of sections containing
the entry symbol and extern (-u) symbols. The search is recursive:
"used" sections found are inserted to the set and their relocations
will be used to search for more "used" sections.

Output relocation sections from input objects if the linker is
creating an relocatable object or if option "-emit-relocs" is
specified.

When the linker encounters a DSO which has a DT_SONAME entry in its
dynamic section, it should use the string referenced by DT_SONAME for
the corresponding DT_NEEDED entry in the output object, instead of
using the DSO filename specified in the command line.

Overhaul of the "symbols" module:
* Use only one hash table instead of three for symbol resolving.
As a result, the symbol resolving code is completely rewritten.
The new method is much simpler and more accurate.
* Build dynamic symbol table before section layout.
* Build dynamic symbol table from the internal symbol table instead
of the import/export symbol table.