Don't leave ARCH unset for mips.

CMake's language is a little dumb about string interpolation. Set it to
"generic", which is the value OPENSSL_NO_ASM uses.

Change-Id: Id98a0309e24465f10bcd7dab4a2000d1038edac0
Reviewed-on: https://boringssl-review.googlesource.com/13261
Reviewed-by: Kenny Root <kroot@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index fbee64a..e15df7a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -181,6 +181,7 @@
   set(ARCH "aarch64")
 elseif (${CMAKE_SYSTEM_PROCESSOR} STREQUAL "mips")
   # Just to avoid the “unknown processor” error.
+  set(ARCH "generic")
 elseif (${CMAKE_SYSTEM_PROCESSOR} STREQUAL "ppc64le")
   set(ARCH "ppc64le")
 else()