Un-defining EIGEN_HAS_CONSTEXPR on the HIP platform

The Eigen unit-tests started failing on the HIP/ROCm platform, after the following commit

e7b8643d

In file included from /home/rocm-user/eigen/test/main.h:360:
In file included from /home/rocm-user/eigen/Eigen/QR:11:
In file included from /home/rocm-user/eigen/Eigen/Core:162:
/home/rocm-user/eigen/Eigen/src/Core/util/Meta.h:300:17: error: constexpr function never produces a constant expression [-Winvalid-constexpr]
  static float (max)() {
                ^
/home/rocm-user/eigen/Eigen/src/Core/util/Meta.h:304:12: note: non-constexpr function '__int_as_float' cannot be used in a constant expression
    return HIPRT_MAX_NORMAL_F;
           ^
/home/rocm-user/eigen/Eigen/src/Core/arch/HIP/hcc/math_constants.h:14:28: note: expanded from macro 'HIPRT_MAX_NORMAL_F'
#define HIPRT_MAX_NORMAL_F __int_as_float(0x7f7fffff)
                           ^
/opt/rocm/hip/include/hip/hcc_detail/device_functions.h:913:32: note: declared here
__device__ static inline float __int_as_float(int x) {
                               ^

The problem seems to that some of the constants defined in the HIP math_constants.h have a call to __int_as_float routine which is not declared constexpr in the HIP runtime header file.

Working around this issue for now, be skipping the const_expr support (enabled via the above commit) on HIP

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@rmlarsen1 @cantonios same issue probably applies to the CUDA platform as well.