[scudo] Allow for non-Android Shared TSD platforms, part 2

Summary:
Follow up to D38826.

We introduce `pthread_{get,set}specific` versions of `{get,set}CurrentTSD` to
allow for non Android platforms to use the Shared TSD model.
We now allow `SCUDO_TSD_EXCLUSIVE` to be defined at compile time.

A couple of things:
- I know that `#if SANITIZER_ANDROID` is not ideal within a function, but in
  the end I feel it looks more compact and clean than going the .inc route; I
  am open to an alternative if anyone has one;
- `SCUDO_TSD_EXCLUSIVE=1` requires ELF TLS support (and not emutls as this uses
  malloc). I haven't found anything to enforce that, so it's currently not
  checked.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D38854

llvm-svn: 315751