[NFC][compiler-rt][hwasan] Move GetCurrentThread to hwasan.cpp

[leonardchan]

Summary

We can reuse the same implementation for getting the current thread on fuchsia.

compiler-rt/lib/hwasan/hwasan.cpp

void HwasanTagMismatch(uptr addr, uptr access_info, uptr *registers_frame,
   else
     ai.size = 1 << (access_info & 0xf);
 
   HandleTagMismatch(ai, (uptr)__builtin_return_address(0),
                     (uptr)__builtin_frame_address(0), nullptr, registers_frame);
   __builtin_unreachable();
 }
 
+Thread *GetCurrentThread() {
+  uptr *ThreadLongPtr = GetCurrentThreadLongPtr();
+  if (UNLIKELY(*ThreadLongPtr == 0))
+    return nullptr;
+  auto *R = (StackAllocationsRingBuffer *)ThreadLongPtr;
+  return hwasanThreadList().GetThreadByBufferAddress((uptr)R->Next());
+}
+
 } // namespace __hwasan
 
 using namespace __hwasan;
 
 void __sanitizer::BufferedStackTrace::UnwindImpl(
     uptr pc, uptr bp, void *context, bool request_fast, u32 max_depth) {
   Thread *t = GetCurrentThread();
   if (!t) {

compiler-rt/lib/hwasan/hwasan_linux.cpp

if (*(uptr *)get_android_tls_ptr() != kMagicValue) {
     Die();
   }
   *tls_ptr = old_value;
 }
 #else
 void AndroidTestTlsSlot() {}
 #endif
 
-Thread *GetCurrentThread() {
-  uptr *ThreadLongPtr = GetCurrentThreadLongPtr();
-  if (UNLIKELY(*ThreadLongPtr == 0))
-    return nullptr;
-  auto *R = (StackAllocationsRingBuffer *)ThreadLongPtr;
-  return hwasanThreadList().GetThreadByBufferAddress((uptr)R->Next());
-}
-
 static AccessInfo GetAccessInfo(siginfo_t *info, ucontext_t *uc) {
   // Access type is passed in a platform dependent way (see below) and encoded
   // as 0xXY, where X&1 is 1 for store, 0 for load, and X&2 is 1 if the error is
   // recoverable. Valid values of Y are 0 to 4, which are interpreted as
   // log2(access_size), and 0xF, which means that access size is passed via
   // platform dependent register (see below).
 #if defined(__aarch64__)
   // Access type is encoded in BRK immediate as 0x900 + 0xXY. For Y == 0xF,