Remove __ASSUME_SET_ROBUST_LIST

This patch removes __ASSUME_SET_ROBUST_LIST usage and assumes that
kernel will correctly return if it supports or not
futex_atomic_cmpxchg_inatomic.

On minimum supported kernel (v3.2 and v2.6.32 for x86) kernel has:

2418 SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
2419                 size_t, len)
2420 {
2421         if (!futex_cmpxchg_enabled)
2422                 return -ENOSYS;

The patch also adds the __set_robust_list_avail runtime check for all
architectures, since for some the syscall may still return ENOSYS if
futex_atomic_cmpxchg_inatomic is not supported (for instance ARM).

Tested on armhf (with 3.8 kernel) and x86_64.

	* nptl/nptl-init.c [__ASSUME_SET_ROBUST_LIST]
	(__set_robust_list_avail): Remove define.
	[__NR_set_robust_list] (__pthread_initialize_minimal_internal):
	Likewise.
	* nptl/pthreadP.h [__ASSUME_SET_ROBUST_LIST]
	(__set_robust_list_avail): Likewise.
	* nptl/pthread_create.c
	[__NR_set_robust_list && !__ASSUME_SET_ROBUST_LIST]
	(START_THREAD_DEFN): Likewise.
	* nptl/pthread_mutex_init.c [!__ASSUME_SET_ROBUST_LIST]
	(__pthread_mutex_init): Likewise.
	* sysdeps/unix/sysv/linux/arm/kernel-features.h
	(__ASSUME_SET_ROBUST_LIST): Likewise.
	* sysdeps/unix/sysv/linux/kernel-features.h:
	(__ASSUME_SET_ROBUST_LIST): Likewise.
	* sysdeps/unix/sysv/linux/m68k/kernel-features.h:
	(__ASSUME_SET_ROBUST_LIST): Likewise.
	* sysdeps/unix/sysv/linux/mips/kernel-features.h:
	(__ASSUME_SET_ROBUST_LIST): Likewise.
	* sysdeps/unix/sysv/linux/sparc/kernel-features.h:
	(__ASSUME_SET_ROBUST_LIST): Likewise.