[MIPS] Adding atomic ops for MIPS architecture.

Initial commit for atomic operations on MIPS architecture.

BUG=https://code.google.com/p/chromium/issues/detail?id=130022
TEST=make chrome


Review URL: https://chromiumcodereview.appspot.com/10448043

git-svn-id: http://src.chromium.org/svn/trunk/src/build@142663 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

build_config.h

@@ -102,6 +102,12 @@
 #define WCHAR_T_IS_UNSIGNED 1
 #elif defined(__pnacl__)
 #define ARCH_CPU_32_BITS 1
+#elif defined(__MIPSEL__)
+#define ARCH_CPU_MIPS_FAMILY 1
+#define ARCH_CPU_MIPSEL 1
+#define ARCH_CPU_32_BITS 1
+#define ARCH_CPU_LITTLE_ENDIAN 1
+#define WCHAR_T_IS_UNSIGNED 0
 #else
 #error Please add support for your architecture in build/build_config.h
 #endif