Rollup merge of #152018 - clarfonthey:bigint-tracking-issues, r=scottmcm

Move bigint helper tracking issues

Closes rust-lang/rust#85532.

This splits the remainder of the `bigint_helper_methods` tracking issue into three issues:

* `signed_bigint_helpers`: rust-lang/rust#151989
* `widening_mul`: rust-lang/rust#152016
* `const_unsigned_bigint_helpers`: rust-lang/rust#152015

Note that the existing stable methods were merged under the `unsigned_bigint_helpers` feature as part of rust-lang/rust#144494.

Author: jhpratt

crates/core_arch/src/lib.rs

@@ -33,7 +33,6 @@
     f16,
     aarch64_unstable_target_feature,
     target_feature_inline_always,
-    bigint_helper_methods,
     funnel_shifts,
     avx10_target_feature,
     const_trait_impl,