[ELF] Fix uninitialized variables

Properly initialize _exportDynamic in ELFLinkingContext and an ELF_Sym object
created in CRuntimeFile with default values.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219380 91177308-0d34-0410-b5e6-96231b3b80d8