[libcxx] Update 128-bit-atomics feature test (#83841)

The `128-bit-atomics` libcxx feature is incorrectly named because tests
that are Xfailed with it is really using `int[128]`. Additionally,
because toolchain support for that feature is determined based on a much
smaller size (`char[16]`), tests would execute incorrectly without
required toolchain support.

So, rename `128-bit-atomics` as `1024-bit-atomics`, and use an
appropriate type to check for the presence of the feature.