MB-8085: detect x86_64-unknown-linux-gnu as GNU/Linux

I.e. so that we can have optimized dir_size implementation.

Looks like our rpm packages build erlang in a way that sets
system_architecture to this value.

Change-Id: I4a3f69b5c8e8a54fd9feaf5830e48e386a809c89
Reviewed-on: http://review.couchbase.org/25643
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
mark

Change-Id: Ic2440264f3fe51b3e6ee5eb70581ffb07d63b12e
mark

Change-Id: I46e219004028e3b0083798bbf7a41ab8b2847b8e