Workaround 'unknown attribute __alloc_size__' warning for Clang 3.5 (trunk)

* include/gc_config_macros.h (GC_ATTR_ALLOC_SIZE): Define to empty if
clang 3.5.0 (workaround "unknown attribute '__alloc_size__' ignored"
warning in clang3.5 (trunk)).
Fix missing error handling of pthreads_mutex_init and cond_wait

* include/private/darwin_semaphore.h (sem_init): Destroy sem->mutex if
sem->cond initialization failed.
* include/private/darwin_semaphore.h (sem_post): Ignore
pthread_mutex_unlock result in case of pthread_cond_signal.
* include/private/darwin_semaphore.h (sem_wait): Unlock mutex and
return error (-1) if pthread_cond_wait failed.
* include/private/darwin_semaphore.h (sem_init): If pshared then
return -1 (with the appropriate errno code set) instead of ABORT.
* include/private/darwin_semaphore.h (sem_init, sem_post, sem_wait):
Treat non-zero value returned by pthread functions as error (instead
of only negative values).
* include/private/darwin_semaphore.h (sem_init, sem_post, sem_wait):
Reformat code.
* misc.c (GC_init): Abort (with the appropriate message) if
pthread_mutex[attr]_init failed (SN_TARGET_PS3 case only).
* specific.c (GC_key_create_inner): If pthread_mutex_init failed then
return its error code.
Fix __alloc_size__ availability detection (Clang)

Since __clang_major__/__clang_minor__ etc. are vendor dependent values,
we cannot implement the feature detection based on it.
For example, Apple clang versioning is different from the FreeBSD clang.
(At this time, Apple clang version is "6.0 (clang-600.0.51)" and
__clang_major__ is 6.)
Instead of this, we can use the clang feature detection macro,
__has_attribute.

* include/gc_config_macros.h (GC_ATTR_ALLOC_SIZE): Replace predefined
__clang_major/minor__ testing with __has_attribute() one (in case of
clang).