Add support for emitting the symbol table (and its string table) of the

Add support for emitting the symbol table (and its string table) of the
module to the ELF file.  Test it by adding support for emitting common
symbols.  This allows us to compile this:

%X = weak global int 0
%Y = weak global int 0
%Z = weak global int 0

to an elf file that 'readelf's this:

Symbol table '.symtab' contains 4 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000004     4 OBJECT  GLOBAL DEFAULT  COM X
     2: 00000004     4 OBJECT  GLOBAL DEFAULT  COM Y
     3: 00000004     4 OBJECT  GLOBAL DEFAULT  COM Z

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22343 91177308-0d34-0410-b5e6-96231b3b80d8