[Patch][libstdc++] Add constexpr specifier to function

[Deev Patel]

Things like std::atomic<double> are currently unable to be created in a
constexpr context with clang 18 and c++20. Example compilation error
```
external/com_google_tcmalloc/tcmalloc/parameters.cc:223:17: error: variable
does not have a constant initializer
  223 |     Parameters::peak_sampling_heap_growth_fraction_(1.1);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
external/com_google_tcmalloc/tcmalloc/parameters.cc:222:1: note: required
by 'constinit' specifier here
  222 | ABSL_CONST_INIT std::atomic<double>
      | ^~~~~~~~~~~~~~~
external/com_google_absl/absl/base/attributes.h:743:25: note: expanded from
macro 'ABSL_CONST_INIT'
  743 | #define ABSL_CONST_INIT constinit
      |                         ^~~~~~~~~
external/local_config_clang/crosstool/extra_tools/lib/gcc/x86_64-linux-gnu/14.1.0/../../../../include/c++/14.1.0/bits/atomic_base.h:1286:9:
note: non-constexpr function '__clear_padding<double>' cannot be used in a
constant expression
 1286 |       { __atomic_impl::__clear_padding(_M_fp); }
      |         ^
external/local_config_clang/crosstool/extra_tools/lib/gcc/x86_64-linux-gnu/14.1.0/../../../../include/c++/14.1.0/atomic:1644:38:
note: in call to '__atomic_float(1.100000e+00)'
 1644 |       atomic(double __fp) noexcept : __atomic_float<double>(__fp)
```

This patch adds the necessary constexpr specifier.

- Deev Patel

diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index 062f1549740..5e89f66f620 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -968,7 +968,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }

     template<typename _Tp>
-      _GLIBCXX_ALWAYS_INLINE _Tp*
+      constexpr _GLIBCXX_ALWAYS_INLINE _Tp*
       __clear_padding(_Tp& __val) noexcept
       {
        auto* __ptr = std::__addressof(__val);