Fix missing `EIGEN_CONSTEXPR` pop_macro in `Half`.

`EIGEN_CONSTEXPR` is getting pushed but not popped in `Half.h` if
`EIGEN_HAS_ARM64_FP16_SCALAR_ARITHMETIC` is defined.

Eigen/src/Core/arch/Default/Half.h

diff --git a/Eigen/src/Core/arch/Default/Half.h b/Eigen/src/Core/arch/Default/Half.h
index bf40814..0bc1e9d 100644
--- a/Eigen/src/Core/arch/Default/Half.h
+++ b/Eigen/src/Core/arch/Default/Half.h
@@ -774,7 +774,7 @@
 
 } // end namespace Eigen
 
-#if defined(EIGEN_HAS_GPU_FP16)
+#if defined(EIGEN_HAS_GPU_FP16) || defined(EIGEN_HAS_ARM64_FP16_SCALAR_ARITHMETIC)
   #pragma pop_macro("EIGEN_CONSTEXPR")
 #endif