Introduce a backport of C11 atomics

This introduces a backport of C11 atomics.  It has four implementations; ranked
in order of preference, they are:
- GCC/Clang __atomic builtins
- GCC/Clang __sync builtins
- MSVC _Interlocked builtins
- C11 atomics, from <stdatomic.h>

The primary advantages are:
- Close adherence to the standard API gives us a defined memory model.
- Type safety: atomic objects are now separate types from non-atomic ones, so
  that it's impossible to mix up atomic and non-atomic updates (which is
  undefined behavior that compilers are starting to take advantage of).
- Efficiency: we can specify ordering for operations, avoiding fences and
  atomic operations on strongly ordered architectures (example:
  `atomic_write_u32(ptr, val);` involves a CAS loop, whereas
  `atomic_store(ptr, val, ATOMIC_RELEASE);` is a plain store.

This diff leaves in the current atomics API (implementing them in terms of the
backport).  This lets us transition uses over piecemeal.

Testing:
This is by nature hard to test. I've manually tested the first three options on
Linux on gcc by futzing with the #defines manually, on freebsd with gcc and
clang, on MSVC, and on OS X with clang.  All of these were x86 machines though,
and we don't have any test infrastructure set up for non-x86 platforms.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -30,16 +30,13 @@
 #undef LG_VADDR
 
 /* Defined if C11 atomics are available. */
-#undef JEMALLOC_C11ATOMICS
+#undef JEMALLOC_C11_ATOMICS
 
-/* Defined if the equivalent of FreeBSD's atomic(9) functions are available. */
-#undef JEMALLOC_ATOMIC9
+/* Defined if GCC __atomic atomics are available. */
+#undef JEMALLOC_GCC_ATOMIC_ATOMICS
 
-/*
- * Defined if OSAtomic*() functions are available, as provided by Darwin, and
- * documented in the atomic(3) manual page.
- */
-#undef JEMALLOC_OSATOMIC
+/* Defined if GCC __sync atomics are available. */
+#undef JEMALLOC_GCC_SYNC_ATOMICS
 
 /*
  * Defined if __sync_add_and_fetch(uint32_t *, uint32_t) and