[ELF] - Re-commit r324322 "Use...

[ELF] - Re-commit r324322 "Use InitTargetOptionsFromCodeGenFlags/ParseCommandLineOptions for parsing LTO options.".

With fix:
Keep logic that ignores -plugin-opt=mcpu=x86-64 -plugin-opt=thinlto,
add checks for those to testcases.

Original commit message:

[ELF] - Use InitTargetOptionsFromCodeGenFlags/ParseCommandLineOptions for parsing LTO options.

gold plugin uses InitTargetOptionsFromCodeGenFlags + 
ParseCommandLineOptions for parsing LTO options.
Patch do the same change for LLD.

Such change helps to avoid parsing/whitelisting LTO
plugin options again on linker side, what can help LLD
to automatically support new -plugin-opt=xxx options
passed.

Differential revision: https://reviews.llvm.org/D42733

llvm-svn: 324340