Workaround for emscripten-core/emscripten#27418 (#637)

When linked in debug mode, there is a linker warning about function
signature msimatch which looks like it should either have linked a
completely broken executable or at best one that always crashes if this
codepath is taken. However, due to strangeness the linked executable
works correctly. We don't like warnings so we fix it anyways.

emscripten-core/emscripten#27418

Author: antocuni

spy/libspy/src/operator.c

@@ -126,7 +126,11 @@ spy_operator$f32_pow(float x, float y) {
         spy_panic("ZeroDivisionError", "0.0 cannot be raised to a negative power",
                   __FILE__, __LINE__);
     }
-    if (x < 0.0f && isfinite(y) && y != truncf(y)) {
+    // Use __builtin_isfinite instead of isfinite: workaround for
+    // https://github.com/emscripten-core/emscripten/issues/27418
+    // Note: We use multivalue abi when building for wasi/emscripten.
+    // See also: spy/build/flags.py
+    if (x < 0.0f && __builtin_isfinite(y) && y != truncf(y)) {
         spy_panic("ValueError", "math domain error", __FILE__, __LINE__);
     }
     return powf(x, y);