Bug 713517 - Add IPC support for alpha. r=benjamin

ipc/chromium/src/build/build_config.h

@@ -86,6 +86,9 @@
 #elif defined(__s390__)
 #define ARCH_CPU_S390 1
 #define ARCH_CPU_32_BITS 1
+#elif defined(__alpha__)
+#define ARCH_CPU_ALPHA 1
+#define ARCH_CPU_64_BITS 1
 #else
 #error Please add support for your architecture in build/build_config.h
 #endif