Fix LLVM-based Arm64 build.
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 <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: David Turner <dcb93bd72059500c1226ad16fdd58ffe25c9e0ea@google.com>
Presubmit-Ready: David Turner <dcb93bd72059500c1226ad16fdd58ffe25c9e0ea@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Reviewed-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader