API guard every post-21 API.

This was generated mechanically by reverting my recent re-hide commits
(except for mempcpy, but versioner cleaned that up for me anyway),
reverting the commits which removed versioner, then:

```
m out/soong/ndk_headers.timestamp
cp -r out/soong/ndk/sysroot/usr/include/* bionic/libc/include
git -C bionic clean -df
```

Effectively, this has restored the `versioner` processed headers, but
I'm checking the results of that in so we don't have to keep
`versioner` around.

For the NDK, this restores r27 behavior by default. Anyone that's
opted into weak APIs will get the new behavior. I think this is our
best option. Anyone writing code with Android in mind should be using
weak APIs, but any code being lightly ported (and thus using the
default configuration) should not be, and it's those ports where we're
having trouble with collisions.

Bug: https://github.com/android/ndk/issues/2081
Test: None
Change-Id: I370079d27566b0c1543fb5890c958c8d09b05006
diff --git a/libc/include/spawn.h b/libc/include/spawn.h
index f366239..b105754 100644
--- a/libc/include/spawn.h
+++ b/libc/include/spawn.h
@@ -55,6 +55,8 @@
 typedef struct __posix_spawnattr* posix_spawnattr_t;
 typedef struct __posix_spawn_file_actions* posix_spawn_file_actions_t;
 
+
+#if __BIONIC_AVAILABILITY_GUARD(28)
 int posix_spawn(pid_t* _Nullable __pid, const char* _Nonnull __path, const posix_spawn_file_actions_t _Nullable * _Nullable __actions, const posix_spawnattr_t _Nullable * _Nullable __attr, char* const _Nullable __argv[_Nullable], char* const _Nullable __env[_Nullable]) __INTRODUCED_IN(28);
 int posix_spawnp(pid_t* _Nullable __pid, const char* _Nonnull __file, const posix_spawn_file_actions_t _Nullable * _Nullable __actions, const posix_spawnattr_t _Nullable * _Nullable __attr, char* const _Nullable __argv[_Nullable], char* const _Nullable __env[_Nullable]) __INTRODUCED_IN(28);
 
@@ -89,9 +91,15 @@
 int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd, const char* _Nonnull __path, int __flags, mode_t __mode) __INTRODUCED_IN(28);
 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd) __INTRODUCED_IN(28);
 int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd, int __new_fd) __INTRODUCED_IN(28);
+#endif /* __BIONIC_AVAILABILITY_GUARD(28) */
 
+
+
+#if __BIONIC_AVAILABILITY_GUARD(34)
 int posix_spawn_file_actions_addchdir_np(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, const char* _Nonnull __path) __INTRODUCED_IN(34);
 int posix_spawn_file_actions_addfchdir_np(posix_spawn_file_actions_t _Nonnull * _Nonnull __actions, int __fd) __INTRODUCED_IN(34);
+#endif /* __BIONIC_AVAILABILITY_GUARD(34) */
+
 
 __END_DECLS