[ELF] Add -Bsymbolic-non-weak

This adds a new -Bsymbolic option that directly binds all non-weak
symbols. There's a couple of reasons motivating this:
* The new flag will match the default behavior on Mach-O, so you can get
  consistent behavior across platforms.
* We have use cases for which making weak data preemptible is useful,
  but we don't want to pessimize access to non-weak data. (For a large
  internal app, we measured 2000+ data symbols whose accesses would be
  unnecessarily pessimized by `-Bsymbolic-functions`.)

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D158322