common/static_ptr: pass an integer to alignas to fix GCC-11 build failure

common/static_ptr: pass an integer to alignas to fix GCC-11 build failure

GCC-11 fails to compile `alignas(std::bit_ceil(Size))` despite std::bit_ceil()
being marked constexpr in libstdc++11. The compiler doesn't recognize it as a
constant expression, while GCC-12+ and Clang-14+ handle it correctly.

Define the alignment value as a separate constexpr variable before passing it
to alignas() to ensure compatibility with GCC-11.

Fixes compilation issue introduced in commit 73399b05 when std::aligned_storage_t
was replaced with alignas.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>