mem: add @branchHint to indexOfSentinel

also seems to work around aarch64 LLVM miscompilation 🤔

lib/std/mem.zig

@@ -1119,6 +1119,7 @@ pub fn indexOfSentinel(comptime T: type, comptime sentinel: T, p: [*:sentinel]co
 
                     i += @divExact(std.mem.alignForward(usize, start_addr, block_size) - start_addr, @sizeOf(T));
                 } else {
+                    @branchHint(.unlikely);
                     // Would read over a page boundary. Per-byte at a time until aligned or found.
                     // 0.39% chance this branch is taken for 4K pages at 16b block length.
                     //