Avoid hard-coded x86_64 arch in tests/basic/rpm.t

tests/basic/rpm.t hard-codes x86_64 to pick the build-root for mock, causing
errors when called from a different architecture. With this patch, we use
'uname -i' to select the right architecture.

v2: Fixed typo as suggested by Justin Clift.

Change-Id: I07bc2af9317dc315bca460149ea3430071537780
BUG: 962169
Signed-off-by: Jose Castillo <jcastillo@redhat.com>
Reviewed-on: http://review.gluster.org/8214
Reviewed-by: Vikhyat Umrao <vumrao@redhat.com>
Reviewed-by: Justin Clift <justin@gluster.org>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>