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>