Rollup merge of #55845 - nikic:emscripten-clamp-mode, r=alexcrichton

Rollup merge of #55845 - nikic:emscripten-clamp-mode, r=alexcrichton

Set BINARYEN_TRAP_MODE=clamp

This fixes the wasm32-unknown-emscripten test failure mentioned in https://github.com/rust-lang/rust/pull/55626#issuecomment-437084774, by making binaryen operate in clamp rather than trap mode.

The issue is that the current `-Zsaturating-float-casts` implementation uses `fpto[us]i` unconditionally (and selects afterwards), which does not work with trapping implementations of fpto[su]i, which emscripten uses by default.

I've left a FIXME to drop this flag once we have a better solution for saturating casts on the LLVM side.
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