Auto merge of #39086 - aidanhs:aphs-local-rebuild-no-jemalloc, r=alexcrichton

Make rustbuild force_alloc_system rather than relying on stage0

This 'fixes' jemalloc-less local rebuilds, where we tell cargo that we're actually stage1 (this only fixes the rustbuild path, since I wasn't enthusiastic to dive into the makefiles).

There should be one effect from this PR: `--enable-local-rebuild --disable-jemalloc` will successfully build a stage0 std (rather than erroring). Ideally I think it'd be nice to specify an allocator preference in Cargo.toml/cargo command line (used when an allocator must be picked i.e. dylibs, not rlibs), but since that's not possible we can make do with a force_alloc_system feature. Sadly this locks you into a single allocator in the build libstd, making any eventual implementation of #38575 not quite right in this edge case, but clearly not many people exercise the combination of these two flags.

This PR is also a substitute for #37975 I think. The crucial difference is that the feature name here is distinct from the jemalloc feature (reused in the previous PR) - we don't want someone to be forced into alloc_system just for disabling jemalloc!

Fixes #39054

r? @alexcrichton

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