ELF improvements:

Handle large integers, x86_fp80, ConstantAggregateZero, and two more ConstantExpr:
GetElementPtr and IntToPtr
Set SHF_MERGE bit for mergeable strings
Avoid zero initialized strings to be classified as a bss symbol
Don't allow common symbols to be classified as STB_WEAK
Add a constant to be used as a global value offset in data relocations 

llvm-svn: 78476