Default to LLVM 7.0 in CMake build.

Bug b/115344057

Change-Id: Icda7cddb26f747c0ce9d4edc4972f9afa3a5e02b
Reviewed-on: https://swiftshader-review.googlesource.com/c/21509
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8023fd4..a5c4e7c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -73,10 +73,10 @@
     set(DEFAULT_REACTOR_BACKEND "LLVM")
 endif()
 
-set(REACTOR_BACKEND DEFAULT_REACTOR_BACKEND CACHE STRING "JIT compiler back-end used by Reactor")
+set(REACTOR_BACKEND ${DEFAULT_REACTOR_BACKEND} CACHE STRING "JIT compiler back-end used by Reactor")
 set_property(CACHE REACTOR_BACKEND PROPERTY STRINGS LLVM Subzero)
 
-set(REACTOR_LLVM_VERSION "3" CACHE STRING "LLVM version used by Reactor")
+set(REACTOR_LLVM_VERSION "7" CACHE STRING "LLVM version used by Reactor")
 
 # LLVM disallows calling cmake . from the main LLVM dir, the reason is that
 # it builds header files that could overwrite the orignal ones. Here we