Fix LLVM-based Arm64 build.

This changes a CMake FATAL_ERROR message into a WARNING to allow
building SwiftShader for arm64 with an LLVM backend properly.

Otherwise, the CMake configuration aborts prematurely even if
Subzero is not used in the final binaries.

Bug: b/154080675
Change-Id: Ia9ea9896d793071c6d275864955a89a1c262c5d6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43853
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: David Turner <digit@google.com>
Presubmit-Ready: David Turner <digit@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

third_party/subzero/CMakeLists.txt

diff --git a/third_party/subzero/CMakeLists.txt b/third_party/subzero/CMakeLists.txt
index 70eb251..031a6d3 100644
--- a/third_party/subzero/CMakeLists.txt
+++ b/third_party/subzero/CMakeLists.txt
@@ -76,7 +76,7 @@
     )
     set(SUBZERO_TARGET_CPU MIPS32)
 else()
-    message(FATAL_ERROR "Architecture '${ARCH}' not supported by Subzero")
+    message(WARNING "Architecture '${ARCH}' not supported by Subzero")
 endif()
 
 if(WIN32)