build: target x86-64-v3 without floating-point contraction

tests/BuildFlagsTests.cpp

@@ -0,0 +1,17 @@
+#include <cstdio>
+
+#if !defined(__AVX2__) || !defined(__BMI2__) || !defined(__FMA__)
+#error "x86-64 builds target x86-64-v3"
+#endif
+
+int main() {
+  // (1 + 2^-23)^2 = 1 + 2^-22 + 2^-46; only a fused multiply-add keeps the 2^-46.
+  volatile float a = 1.0f + 0x1p-23f;
+  const float result = a * a - (1.0f + 0x1p-22f);
+  if (result != 0.0f) {
+    std::fprintf(stderr, "BuildFlagsTests: failed: a * a - b was contracted (%a)\n", result);
+    return 1;
+  }
+  std::puts("BuildFlagsTests: all cases passed");
+  return 0;
+}