[lld][linkerscript] Fix handling of DEFINED.
Current implementation did not check that symbols is actually defined. Only checked for presence. GNU ld documentation says,
"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."
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D83758