Modify vm code base so that it can be built for --arch=simmips.

runtime/vm/os_linux.cc

Index: runtime/vm/os_linux.cc
===================================================================
--- runtime/vm/os_linux.cc	(revision 17318)
+++ runtime/vm/os_linux.cc	(working copy)
@@ -250,7 +250,8 @@
 word OS::ActivationFrameAlignment() {
 #if defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_X64)
   const int kMinimumAlignment = 16;
-#elif defined(TARGET_ARCH_ARM)
+#elif defined(TARGET_ARCH_ARM) || defined(TARGET_ARCH_MIPS)
+  // TODO(regis): Verify alignment constraints on MIPS.
   const int kMinimumAlignment = 8;
 #else
 #error Unsupported architecture.
@@ -268,7 +269,8 @@
 word OS::PreferredCodeAlignment() {
 #if defined(TARGET_ARCH_IA32) || defined(TARGET_ARCH_X64)
   const int kMinimumAlignment = 16;
-#elif defined(TARGET_ARCH_ARM)
+#elif defined(TARGET_ARCH_ARM) || defined(TARGET_ARCH_MIPS)
+  // TODO(regis): Verify alignment constraints on MIPS.
   const int kMinimumAlignment = 16;
 #else
 #error Unsupported architecture.