Silence clang warning spewed by every .cpp compile

Silence clang warning spewed by every .cpp compile

clang: warning: argument unused during compilation: '-s' [-Wunused-command-line-argument]

Bug: b/123933266
Change-Id: I809458fa08b9b6b3af7d31c19a90206462d0b0c6
Reviewed-on: https://swiftshader-review.googlesource.com/c/24429
Kokoro-Presubmit: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Ben Clayton <bclayton@google.com>