aarch64: check for LITTLE cores

This is a fix for bug explosion#13 : add logic to setup.py to detect aarch64
LITTLE cores for which BLIS has configure flag BLIS_ARCH="cortexa53".  All other
cores will configure with BLIS_ARCH="cortexa57".

setup.py

@@ -134,6 +134,15 @@ def build_extensions(self):
     def get_arch_name(self):
         if "BLIS_ARCH" in os.environ:
             return os.environ["BLIS_ARCH"]
+        machine = os.uname().machine
+        if machine == "aarch64":
+            # Check if this is a LITTLE core.
+            LITTLE_core = os.system("test 'Cortex-A53' = $(lscpu | grep 'Model name' | awk '{print $3}')")
+            if LITTLE_core == 0:
+                return "cortexa53"
+            else:
+                # Optimize for big cores.
+                return "cortexa57"
         else:
             return "x86_64"