https://swiftshader-review.googlesource.com/c/SwiftShader/+/40788 and
https://swiftshader-review.googlesource.com/c/SwiftShader/+/41968 cause
us to use different compile flags for Subzero's LLVM dependencies than
the rest of SwiftShader. Suppress the warnings that this has unleashed.

Bug: b/132445520
Change-Id: I1f20e32b487035e953c02cd4829460c072e38ac3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41969
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>