Remove guards on constants and types.

The only remaining guard is the one on the posix_madvise constants,
because that's one where we had a bug filed for breaking some 3p code
where the presence of those macros was being used as a proxy for
runtime availability of posix_madvise. That's wrong, but it's in the
wild so we can't do much about it. That wrong code is still in
upstream boost:
https://github.com/boostorg/interprocess/blob/9fc4ee7dd7277d86c691921007856aca99c3c1ac/include/boost/interprocess/mapped_region.hpp#L811-L814

Bug: android/ndk#1652
Test: treehugger
Change-Id: If5762422524e1667014b4b4ce6a2a2700dd4aff0

Author: DanAlbert

libc/include/bits/pthread_types.h

@@ -43,19 +43,15 @@ typedef struct {
 #endif
 } pthread_attr_t;
 
-#if __ANDROID_API__ >= 24
 typedef struct {
 #if defined(__LP64__)
   int64_t __private[4];
 #else
   int32_t __private[8];
 #endif
 } pthread_barrier_t;
-#endif
 
-#if __ANDROID_API__ >= 24
 typedef int pthread_barrierattr_t;
-#endif
 
 typedef struct {
 #if defined(__LP64__)
@@ -91,14 +87,12 @@ typedef struct {
 
 typedef long pthread_rwlockattr_t;
 
-#if __ANDROID_API__ >= 24
 typedef struct {
 #if defined(__LP64__)
   int64_t __private;
 #else
   int32_t __private[2];
 #endif
 } pthread_spinlock_t;
-#endif
 
 typedef long pthread_t;

libc/include/fcntl.h

@@ -93,14 +93,12 @@ __BEGIN_DECLS
 /** Flag for splice(). */
 #define SPLICE_F_GIFT 8
 
-#if __ANDROID_API__ >= 26
 /** Flag for sync_file_range(). */
 #define SYNC_FILE_RANGE_WAIT_BEFORE 1
 /** Flag for sync_file_range(). */
 #define SYNC_FILE_RANGE_WRITE 2
 /** Flag for sync_file_range(). */
 #define SYNC_FILE_RANGE_WAIT_AFTER 4
-#endif
 
 /**
  * [creat(2)](https://man7.org/linux/man-pages/man2/creat.2.html)

libc/include/pthread.h

@@ -70,9 +70,7 @@ enum {
 
 #define PTHREAD_ONCE_INIT 0
 
-#if __ANDROID_API__ >= 24
 #define PTHREAD_BARRIER_SERIAL_THREAD (-1)
-#endif
 
 #if defined(__LP64__)
 #define PTHREAD_STACK_MIN 16384
@@ -246,26 +244,20 @@ int pthread_rwlock_trywrlock(pthread_rwlock_t* _Nonnull __rwlock);
 int pthread_rwlock_unlock(pthread_rwlock_t* _Nonnull __rwlock);
 int pthread_rwlock_wrlock(pthread_rwlock_t* _Nonnull __rwlock);
 
-#if __ANDROID_API__ >= 24
 int pthread_barrierattr_init(pthread_barrierattr_t* _Nonnull __attr) __INTRODUCED_IN(24);
 int pthread_barrierattr_destroy(pthread_barrierattr_t* _Nonnull __attr) __INTRODUCED_IN(24);
 int pthread_barrierattr_getpshared(const pthread_barrierattr_t* _Nonnull __attr, int* _Nonnull __shared) __INTRODUCED_IN(24);
 int pthread_barrierattr_setpshared(pthread_barrierattr_t* _Nonnull __attr, int __shared) __INTRODUCED_IN(24);
-#endif
 
-#if __ANDROID_API__ >= 24
 int pthread_barrier_init(pthread_barrier_t* _Nonnull __barrier, const pthread_barrierattr_t* _Nullable __attr, unsigned __count) __INTRODUCED_IN(24);
 int pthread_barrier_destroy(pthread_barrier_t* _Nonnull __barrier) __INTRODUCED_IN(24);
 int pthread_barrier_wait(pthread_barrier_t* _Nonnull __barrier) __INTRODUCED_IN(24);
-#endif
 
-#if __ANDROID_API__ >= 24
 int pthread_spin_destroy(pthread_spinlock_t* _Nonnull __spinlock) __INTRODUCED_IN(24);
 int pthread_spin_init(pthread_spinlock_t* _Nonnull __spinlock, int __shared) __INTRODUCED_IN(24);
 int pthread_spin_lock(pthread_spinlock_t* _Nonnull __spinlock) __INTRODUCED_IN(24);
 int pthread_spin_trylock(pthread_spinlock_t* _Nonnull __spinlock) __INTRODUCED_IN(24);
 int pthread_spin_unlock(pthread_spinlock_t* _Nonnull __spinlock) __INTRODUCED_IN(24);
-#endif
 
 pthread_t pthread_self(void) __attribute_const__;