[GCC] Build fix for Ubuntu 22.04 after 281895@main

https://bugs.webkit.org/show_bug.cgi?id=277672

Reviewed by NOBODY (OOPS!).

GCC <12 do not feature type '_Float16'.

Check compiler flag '__FLT16_MANT_DIG__' is defined before using '_Float16'.

* Source/WTF/wtf/Float16.h:
(WTF::convertFloat16ToFloat32):
(WTF::convertFloat32ToFloat16):
(WTF::convertFloat64ToFloat16):

Source/WTF/wtf/Float16.h

@@ -42,6 +42,7 @@
 #pragma once
 
 #include <bit>
+#include <cfloat>
 #include <wtf/StdLibExtras.h>
 
 namespace WTF {
@@ -55,9 +56,10 @@ namespace WTF {
  */
 constexpr float convertFloat16ToFloat32(uint16_t h)
 {
-#if CPU(ARM64) || CPU(X86_64)
+#if (CPU(ARM64) || CPU(X86_64)) && defined(__FLT16_MANT_DIG__)
     return static_cast<float>(std::bit_cast<_Float16>(h));
 #else
+
     /*
      * Extend the half-precision floating-point number to 32 bits and shift to the upper part of the 32-bit word:
      *      +---+-----+------------+-------------------+
@@ -176,7 +178,7 @@ constexpr double convertFloat16ToFloat64(uint16_t h)
  */
 constexpr uint16_t convertFloat32ToFloat16(float f)
 {
-#if CPU(ARM64) || CPU(X86_64)
+#if (CPU(ARM64) || CPU(X86_64)) && defined(__FLT16_MANT_DIG__)
     return std::bit_cast<uint16_t>(static_cast<_Float16>(f));
 #else
     const float scale_to_inf = 0x1.0p+112f; // 0x77800000
@@ -204,7 +206,7 @@ constexpr uint16_t convertFloat32ToFloat16(float f)
 // Adopted from V8's DoubleToFloat16, which adopted from https://gist.github.com/rygorous/2156668.
 constexpr uint16_t convertFloat64ToFloat16(double value)
 {
-#if CPU(ARM64) || CPU(X86_64)
+#if (CPU(ARM64) || CPU(X86_64)) && defined(__FLT16_MANT_DIG__)
     return std::bit_cast<uint16_t>(static_cast<_Float16>(value));
 #else
     // uint64_t constants prefixed with kFP64 are bit patterns of doubles.
@@ -284,7 +286,7 @@ constexpr uint16_t convertFloat64ToFloat16(double value)
 #endif
 }
 
-#if CPU(ARM64) || CPU(X86_64)
+#if (CPU(ARM64) || CPU(X86_64)) && defined(__FLT16_MANT_DIG__)
 class Float16 {
 public:
     constexpr Float16() = default;