[FFI] Fix JSON parser/writer for the fast-math flag (apache#18221)

This PR fixes the JSON parser and writer for the support of the
fast-math flag.

Prior to this PR, there would be the following error when compiling
TVM with fast-math. This PR fixes this issue.
```
/home/ruihangl/Workspace/tvm/ffi/src/ffi/extra/json_writer.cc: In static member function ‘static bool tvm::ffi::json::JSONWriter::FastMathSafeIsNaN(double)’:
/home/ruihangl/Workspace/tvm/ffi/src/ffi/extra/json_writer.cc:69:22: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
   69 |     uint64_t bits = *reinterpret_cast<const uint64_t*>(&x);
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/ruihangl/Workspace/tvm/ffi/src/ffi/extra/json_writer.cc: In static member function ‘static bool tvm::ffi::json::JSONWriter::FastMathSafeIsInf(double)’:
/home/ruihangl/Workspace/tvm/ffi/src/ffi/extra/json_writer.cc:84:22: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
   84 |     uint64_t bits = *reinterpret_cast<const uint64_t*>(&x);
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Author: MasterJH5574

src/ffi/extra/json_parser.cc

@@ -298,26 +298,38 @@ class JSONParserContext {
  private:
   static double FastMathSafePosInf() {
 #ifdef __FAST_MATH__
-    const uint64_t inf_bits = 0x7FF0000000000000ULL;
-    return *reinterpret_cast<const double*>(&inf_bits);
+    union {
+      uint64_t from;
+      double to;
+    } u;
+    u.from = 0x7FF0000000000000ULL;  // write "from", read "to"
+    return u.to;
 #else
     return std::numeric_limits<double>::infinity();
 #endif
   }
 
   static double FastMathSafeNegInf() {
 #ifdef __FAST_MATH__
-    const uint64_t inf_bits = 0xFFF0000000000000ULL;
-    return *reinterpret_cast<const double*>(&inf_bits);
+    union {
+      uint64_t from;
+      double to;
+    } u;
+    u.from = 0xFFF0000000000000ULL;  // write "from", read "to"
+    return u.to;
 #else
     return -std::numeric_limits<double>::infinity();
 #endif
   }
 
   static double FastMathSafeNaN() {
 #ifdef __FAST_MATH__
-    const uint64_t nan_bits = 0x7FF8000000000000ULL;
-    return *reinterpret_cast<const double*>(&nan_bits);
+    union {
+      uint64_t from;
+      double to;
+    } u;
+    u.from = 0x7FF8000000000000ULL;  // write "from", read "to"
+    return u.to;
 #else
     return std::numeric_limits<double>::quiet_NaN();
 #endif

src/ffi/extra/json_writer.cc

@@ -66,7 +66,12 @@ class JSONWriter {
     // IEEE 754 standard: https://en.wikipedia.org/wiki/IEEE_754
     // NaN is encoded as all 1s in the exponent and non-zero in the mantissa
     static_assert(sizeof(double) == sizeof(uint64_t), "Unexpected double size");
-    uint64_t bits = *reinterpret_cast<const uint64_t*>(&x);
+    union {
+      double from;
+      uint64_t to;
+    } u;
+    u.from = x;  // write "from", read "to"
+    uint64_t bits = u.to;
     uint64_t exponent = (bits >> 52) & 0x7FF;
     uint64_t mantissa = bits & 0xFFFFFFFFFFFFFull;
     return (exponent == 0x7FF) && (mantissa != 0);
@@ -81,7 +86,12 @@ class JSONWriter {
     // IEEE 754 standard: https://en.wikipedia.org/wiki/IEEE_754
     // Inf is encoded as all 1s in the exponent and zero in the mantissa
     static_assert(sizeof(double) == sizeof(uint64_t), "Unexpected double size");
-    uint64_t bits = *reinterpret_cast<const uint64_t*>(&x);
+    union {
+      double from;
+      uint64_t to;
+    } u;
+    u.from = x;  // write "from", read "to"
+    uint64_t bits = u.to;
     uint64_t exponent = (bits >> 52) & 0x7FF;
     uint64_t mantissa = bits & 0xFFFFFFFFFFFFFull;
     // inf is encoded as all 1s in the exponent and zero in the mantissa