Fix ./build-m5 "Do not know how to make File target `build/aarch64/out/m5'"

The correct m5 target is: submodules/gem5/util/m5/build/arm64/out/m5, but
we were using submodules/gem5/util/m5/build/aarch6/out/m5 instead,
presumably this changed on some gem5 version update, and we didn't notice
because artifacts were leftover in the repo due to in-tree build.

Thanks for EE415-20170470 for proposing the patch at:
#145 (comment)

Fix #145

Author: cirosantilli

common.py

@@ -928,6 +928,8 @@ def join(*paths):
         env['gem5_m5_source_dir'] = join(env['gem5_source_dir'], 'util', 'm5')
         if self.env['arch'] == 'x86_64':
             env['gem5_m5_source_dir_build_arch'] = 'x86'
+        elif self.env['arch'] == 'aarch64':
+            env['gem5_m5_source_dir_build_arch'] = 'arm64'
         else:
             env['gem5_m5_source_dir_build_arch'] = env['arch']
         env['gem5_m5_source_dir_build'] = join(env['gem5_m5_source_dir'], 'build', env['gem5_m5_source_dir_build_arch'], 'out', 'm5')