build: switch submodule to upstream jemalloc and link atomic for musl

[reigadegr]

Summary

Point the jemalloc submodule to the official jemalloc/jemalloc repo instead of the tikv/jemalloc fork. Always link libatomic when building for *-unknown-linux-musl targets to prevent undefined-reference errors like __atomic_exchange_1.

Details

.gitmodules: updated URL to https://github.com/jemalloc/jemalloc. jemalloc-sys/build.rs: extend the existing RISC-V check so that libatomic is also linked for musl targets.

No behavioural changes in the resulting allocator.

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