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>