[lldb] Make the thread_local g_global_boundary accessed from a single file

This makes the compiler generated code for accessing the thread local
variable much simpler (no need for wrapper functions and weak pointers
to potential init functions), and can avoid toolchain bugs regarding how
to access TLS variables.

In particular, this fixes LLDB when built with current GCC/binutils for
MinGW, see https://github.com/msys2/MINGW-packages/issues/8868.

Differential Revision: https://reviews.llvm.org/D111779