LLVM 7.0 properly supports ARM (unlike the LLVM 3.0 version we used
before), including the features needed by our Vulkan implementation
(unlike Subzero).

Bug: b/115344057
Change-Id: Ic9155b644ab649597219093c3d757862c60a9fa0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35228
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>