[arm] Correctly detect ARMv8 platforms with old kernels.

src/base/cpu.cc

Index: src/base/cpu.cc
diff --git a/src/base/cpu.cc b/src/base/cpu.cc
index 777f379bae90e1e3f0155e8ec9ae5f0fe87558bc..12a38819197a90aa4d91dfbbef86aa64cb1f3455 100644
--- a/src/base/cpu.cc
+++ b/src/base/cpu.cc
@@ -468,7 +468,12 @@ CPU::CPU()
     char* end;
     architecture_ = strtol(architecture, &end, 10);
     if (end == architecture) {
-      architecture_ = 0;
+      // Kernels older than 3.18 report "CPU architecture: AArch64" on ARMv8.
+      if (strcmp(architecture, "AArch64") == 0) {
+        architecture_ = 8;
+      } else {
+        architecture_ = 0;
+      }
     }
     delete[] architecture;