Stabilize new_zeroed_alloc

[thaliaarchi]

The corresponding new_uninit and new_uninit_slice functions were stabilized in #129401, but the zeroed counterparts were left for later out of a desire to stabilize only the minimal set. These functions are straightforward mirrors of the uninit functions and well-established. Since no blockers or design questions have surfaced in the past year, I think it's time to stabilize them.

Tracking issue: #129396

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[thaliaarchi]

@jieyouxu Are the bootstrap gates for rustc_index correct with the redesigned bootstrap sequence? It needs to be compilable in four ways: with feature = "nightly" by stage0, stage1, or from crates.io, or without feature = "nightly" from crates.io.