Fix hb_atomic_* variants based on C++11 atomics

I stumbled over this when trying to upgrade the version of HarfBuzz used by
LibreOffice to 3.2.1 (see <https://gerrit.libreoffice.org/plugins/gitiles/core/
+/b7ddc514bff9bdf682abae537f990aa01dc2c0fb%5E!/> "Upgrade to latest
HarfBuzz 2.3.1"), where building with MSVC 2017 failed like

> c:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows\workdir\unpackedtarball\harfbuzz\src\hb-atomic.hh(272): error C2440: 'reinterpret_cast': cannot convert from 'const int *' to 'std::atomic<int> *'
> c:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows\workdir\unpackedtarball\harfbuzz\src\hb-atomic.hh(272): note: Conversion loses qualifiers
> c:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows\workdir\unpackedtarball\harfbuzz\src\hb-atomic.hh(272): error C2227: left of '->load' must point to class/struct/union/generic type

(see <https://ci.libreoffice.org/job/gerrit_windows/29916/>).

I added all the necessary "const" to make building of HarfBuzz 2.3.1 with
MSVC 2017 succeed for me.  There may be more missing at least conceptually.

src/hb-atomic.hh

@@ -85,11 +85,11 @@ _hb_atomic_ptr_impl_cmplexch (const void **P, const void *O_, const void *N)
 #define hb_atomic_int_impl_add(AI, V)		(reinterpret_cast<std::atomic<int> *> (AI)->fetch_add ((V), std::memory_order_acq_rel))
 #define hb_atomic_int_impl_set_relaxed(AI, V)	(reinterpret_cast<std::atomic<int> *> (AI)->store ((V), std::memory_order_relaxed))
 #define hb_atomic_int_impl_set(AI, V)		(reinterpret_cast<std::atomic<int> *> (AI)->store ((V), std::memory_order_release))
-#define hb_atomic_int_impl_get_relaxed(AI)	(reinterpret_cast<std::atomic<int> *> (AI)->load (std::memory_order_relaxed))
-#define hb_atomic_int_impl_get(AI)		(reinterpret_cast<std::atomic<int> *> (AI)->load (std::memory_order_acquire))
+#define hb_atomic_int_impl_get_relaxed(AI)	(reinterpret_cast<std::atomic<int> const *> (AI)->load (std::memory_order_relaxed))
+#define hb_atomic_int_impl_get(AI)		(reinterpret_cast<std::atomic<int> const *> (AI)->load (std::memory_order_acquire))
 
 #define hb_atomic_ptr_impl_set_relaxed(P, V)	(reinterpret_cast<std::atomic<void*> *> (P)->store ((V), std::memory_order_relaxed))
-#define hb_atomic_ptr_impl_get_relaxed(P)	(reinterpret_cast<std::atomic<void*> *> (P)->load (std::memory_order_relaxed))
+#define hb_atomic_ptr_impl_get_relaxed(P)	(reinterpret_cast<std::atomic<void*> const *> (P)->load (std::memory_order_relaxed))
 #define hb_atomic_ptr_impl_get(P)		(reinterpret_cast<std::atomic<void*> *> (P)->load (std::memory_order_acquire))
 static inline bool
 _hb_atomic_ptr_impl_cmplexch (const void **P, const void *O_, const void *N)