Use the same tls section name as msvc.

We currently error in clang with:
"error: thread-local storage is unsupported for the current target", but we
can start to get the llvm level ready.

When compiling

template<typename T>
struct foo {
  static __declspec(thread) int bar;
};
template<typename T>
__declspec(therad) int foo<T>::bar;
template struct foo<int>;

msvc produces

SECTION HEADER #3
   .tls$ name
       0 physical address
       0 virtual address
       4 size of raw data
     12F file pointer to raw data (0000012F to 00000132)
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
C0301040 flags
         Initialized Data
         COMDAT; sym= "public: static int foo<int>::bar" (?bar@?$foo@H@@2HA)
         4 byte align
         Read Write

gcc produces a ".data$__emutls_v.<symbol>" for the testcase with
__declspec(thread) replaced with thread_local.