aarch64: add HWCAP_ATOMICS to HWCAP_IMPORTANT

[Szabolcs Nagy]

This enables searching shared libraries in atomics/ when the hardware
supports LSE atomics of armv8.1 so one can provide optimized variants
of libraries in a portable way.

LSE atomics does not affect library abi, the new instructions can
interoperate with old ones.

I'm not familiar with how this feature of the dynamic linker is used
in practice by distros or others so comments are welcome.

2018-04-19  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h (HWCAP_IMPORTANT): Add
	HWCAP_ATOMICS.
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