Tag GC malloc routines with alloc_size attribute for Clang 3.2+

* include/gc_config_macros.h (GC_ATTR_ALLOC_SIZE): Define as alloc_size
attribute for Clang 3.2+ as well.
Workaround 'unresolved __tls_get_addr' error for Android NDK Clang/x86

* include/private/thread_local_alloc.h (USE_COMPILER_TLS): Do not use
for Android NDK Clang (to workaround "unresolved __tls_get_addr" linker
error reported both by clang/x86 and clang/mips).