Rollup merge of #61586 - alexcrichton:asmjs-no-assertions, r=pietroalbini

ci: Disable LLVM/debug assertions for asmjs builder

This shaves of 50 minutes of cycle time on Azure and will likely also
save a significant chunk of time on Travis. The assertions here aren't
really buying us much over other builders with assertions already
enabled, so let's disable them for this builder.

cc #61185