[ELF] - Implemented --defsym option.

gnu ld description of option is:

--defsym=symbol=expression
Create a global symbol in the output file, containing the absolute address given 
by expression. You may use this option as many times as necessary to define multiple
symbols in the command line. A limited form of arithmetic is supported for the 
expression in this context: you may give a hexadecimal constant or the name of an
existing symbol, or use "+" and "-" to add or subtract hexadecimal constants or 
symbols. If you need more elaborate expressions, consider using the linker command
language from a script. Note: there should be no white space between symbol, 
the equals sign ("="), and expression.

In compare with D32082, this patch does not support math expressions and absolute
symbols. It implemented via code similar to --wrap. That covers 1 of 3 possible
--defsym cases.

Differential revision: https://reviews.llvm.org/D32171

llvm-svn: 301391