[sanitizer] Change the way GetThreadStackAndTls() obtains the thread descriptor address.

Instead of using arch_prctl(ARCH_GET_FS), read the address from the
tread descriptor itself. This lets us avoid sandboxing issues. Also,
GetThreadStackAndTls() can now be implemented on i386.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182853 91177308-0d34-0410-b5e6-96231b3b80d8