[MIPS] Add CONFIG_32BIT to kernel_default_arch_macros for MIPS.
Some of the required structure definitons are defined under this macro.

Change-Id: I7a9a986e371381697dcb56bd8ea362856ffa7509
diff --git a/libc/kernel/tools/defaults.py b/libc/kernel/tools/defaults.py
index 86a14fd..7eeffb4 100644
--- a/libc/kernel/tools/defaults.py
+++ b/libc/kernel/tools/defaults.py
@@ -45,6 +45,7 @@
 kernel_default_arch_macros = {
     "arm": {},
     "x86": {"__i386__": "1", "CONFIG_X86_32": "1"},
+    "mips": {"CONFIG_32BIT":"1"},
     }
 
 # Replace tokens in the output according to this mapping