[scudo] Allow for non-Android Shared TSD platforms, part 1

Summary:
This first part just prepares the grounds for part 2 and doesn't add any new
functionality. It mostly consists of small refactors:
- move the `pthread.h` include higher as it will be used in the headers;
- use `errno.h` in `scudo_allocator.cpp` instead of the sanitizer one, update
  the `errno` assignments accordingly (otherwise it creates conflicts on some
  platforms due to `pthread.h` including `errno.h`);
- introduce and use `getCurrentTSD` and `setCurrentTSD` for the shared TSD
  model code;

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits, srhines

Differential Revision: https://reviews.llvm.org/D38826

llvm-svn: 315583