[ELF] - Linkerscript: implement DEFINED() command.

DEFINED(symbol)
Return 1 if symbol is in the linker global symbol table and is defined before
the statement using DEFINED in the script, otherwise return 0.

Can be used to define default values for symbols. Found it in the wild.

Differential revision: https://reviews.llvm.org/D24858

llvm-svn: 282245