Don't needlessly build LLVM

If the Reactor backend is Subzero, there's no need to build LLVM. This
is accomplished by excluding the LLVM target from the "all" target.

Bug: b/151250656
Change-Id: I26dc44921cc44d379c619700c6dd03f6fe94ee27
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42229
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4b97f37..37789cc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1535,7 +1535,7 @@
     )
 endif()
 
-add_library(llvm STATIC ${LLVM_LIST})
+add_library(llvm STATIC EXCLUDE_FROM_ALL ${LLVM_LIST})
 set_target_properties(llvm PROPERTIES
     POSITION_INDEPENDENT_CODE 1
     INCLUDE_DIRECTORIES "${LLVM_INCLUDE_DIR}"