[SanitizerCoverage][Fuchsia] Make TracePcGuardController linker-initialized

[mcgrathr]

Summary

It was always intended to be.

[phosek]

LGTM

compiler-rt/trunk/lib/sanitizer_common/sanitizer_coverage_fuchsia.cc

 // TODO(mcgrathr): Move the constant into a header shared with other impls.
 constexpr u64 Magic64 = 0xC0BFFFFFFFFFFF64ULL;
 static_assert(SANITIZER_WORDSIZE == 64, "Fuchsia is always LP64");
 
 constexpr const char kSancovSinkName[] = "sancov";
 
 // Collects trace-pc guard coverage.
 // This class relies on zero-initialization.
-class TracePcGuardController {
+class TracePcGuardController final {
  public:
   // For each PC location being tracked, there is a u32 reserved in global
   // data called the "guard".  At startup, we assign each guard slot a
   // unique index into the big results array.  Later during runtime, the
   // first call to TracePcGuard (below) will store the corresponding PC at
   // that index in the array.  (Each later call with the same guard slot is
   // presumed to be from the same PC.)  Then it clears the guard slot back
   // to zero, which tells the compiler not to bother calling in again.  At

 
  private:
   // We map in the largest possible view into the VMO: one word
   // for every possible 32-bit index value.  This avoids the need
   // to change the mapping when increasing the size of the VMO.
   // We can always spare the 32G of address space.
   static constexpr size_t MappingSize = sizeof(uptr) << 32;
 
-  BlockingMutex setup_lock_;
-  uptr *array_;
-  u32 next_index_;
-  zx_handle_t vmo_;
-  char vmo_name_[ZX_MAX_NAME_LEN];
+  BlockingMutex setup_lock_{LINKER_INITIALIZED};
+  uptr *array_{};
+  u32 next_index_{};
+  zx_handle_t vmo_{};
+  char vmo_name_[ZX_MAX_NAME_LEN]{};
 
   size_t DataSize() const { return next_index_ * sizeof(uintptr_t); }
 
   u32 Setup(u32 num_guards) {
     BlockingMutexLock locked(&setup_lock_);
     DCHECK(common_flags()->coverage);
 
     if (next_index_ == 0) {