Rollup merge of #147457 - the8472:slice_fill_memset2, r=RalfJung,joboet

specialize slice::fill to use memset when possible

It helps const eval performance rust-lang#4616, debug builds and the gcc backend.

Previously attempted in rust-lang/rust#83245 but reverted due to unsoundness rust-lang/rust#87891 around potentially-uninitialized types. This PR only handles primitives where the problem does not arise.

split off from rust-lang/rust#147294

Author: Zalathar