Use compiler-provided assume builtins when available

There are several benefits to this:
1. It's cleaner and more reliable to use the builtin to
   inform the compiler of assumptions instead of hoping that the
   optimizer understands your intentions.
2. `clang` will warn you if any of your assumptions would produce
   side-effects (which the compiler will discard). [This blog post](https://fastcompression.blogspot.com/2019/01/compiler-checked-contracts.html)
   by Yann Collet highlights that a hazard of using the
   `unreachable()`-based method of signaling assumptions is that it
   can sometimes result in additional instructions being generated (see
   [this Godbolt link](https://godbolt.org/z/lKNMs3) from the blog post
   for an example).

include/jemalloc/internal/arena_inlines_b.h

@@ -378,7 +378,7 @@ arena_dalloc(tsdn_t *tsdn, void *ptr, tcache_t *tcache,
 	if (caller_alloc_ctx != NULL) {
 		alloc_ctx = *caller_alloc_ctx;
 	} else {
-		util_assume(!tsdn_null(tsdn));
+		util_assume(tsdn != NULL);
 		emap_alloc_ctx_lookup(tsdn, &arena_emap_global, ptr,
 		    &alloc_ctx);
 	}

include/jemalloc/internal/util.h

@@ -65,12 +65,19 @@ get_errno(void) {
 #endif
 }
 
-JEMALLOC_ALWAYS_INLINE void
+#ifdef _MSC_VER
-util_assume(bool b) {
+#define util_assume __assume
-	if (!b) {
+#elif defined(__clang__) && (__clang_major__ > 3 || \
-		unreachable();
+    (__clang_major__ == 3 && __clang_minor__ >= 6))
-	}
+#define util_assume __builtin_assume
-}
+#else
+#define util_assume(expr)		\
+	do {				\
+		if (!(expr)) {		\
+			unreachable();	\
+		}			\
+	} while(0)
+#endif
 
 /* ptr should be valid. */
 JEMALLOC_ALWAYS_INLINE void