arm: Re-enable PI futex support for ARM kernels >= 3.14.3

ARM linux kernels before 3.14.3 may or may not support
futex_atomic_cmpxchg_inatomic depending on the kernel configuration (e.g.
CONFIG_CPU_USE_DOMAINS && CONFIG_SMP configuration was not supported)

Starting with 3.14.3 the linux kernel unconditionally enables support for
ARM, and this re-enables the relevant __ASSUME_* macros.

Tested on ARM both with kernels >= 3.14.3 and older kernels.

	* sysdeps/unix/sysv/linux/arm/kernel-features.h
	[__LINUX_KERNEL_VERSION >= 0x030E03] (__ASSUME_FUTEX_LOCK_PI): Do
	not undefine.
	[__LINUX_KERNEL_VERSION >= 0x030E03] (__ASSUME_REQUEUE_PI):
	Likewise.
	[__LINUX_KERNEL_VERSION >= 0x030E03] (__ASSUME_SET_ROBUST_LIST):
	Likewise.