Add support for 32b MIPS architecture

* LLVM reactor backend: requires LLVM 7.0
* Subzero reactor backend: unittests hit unimplemented
  TargetMIPS32::lowerShuffleVector()

Bug: b/117854176
Change-Id: Ie58e3e438db6f1b442b05efecf9b645aff82321a
Reviewed-on: https://swiftshader-review.googlesource.com/c/21748
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Milko Leporis <milko.leporis@mips.com>
diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index ca0c0ae..c08572c 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -822,6 +822,8 @@
 			static const char arch[] = "arm64";
 		#elif defined(__arm__)
 			static const char arch[] = "arm";
+		#elif defined(__mips__)
+			static const char arch[] = "mipsel";
 		#else
 		#error "unknown architecture"
 		#endif