[ELF] - Define linkerscript symbols early.

Currently symbols assigned or created by linkerscript are not processed early
enough. As a result it is not possible to version them or assign any other flags/properties.

Patch creates Defined symbols for -defsym and linkerscript symbols early,
so that issue from above can be addressed.

It is based on Rafael Espindola's version of D38239 patch.

Fixes PR34121.

Differential revision: https://reviews.llvm.org/D41987

llvm-svn: 323729