Auto merge of #56825 - alexcrichton:demangle-mem, r=nikomatsakis
std: Activate compiler_builtins `mem` feature for no_std targets
This was an accidental regression from #56092, but for `no_std` targets
being built and distributed we want to be sure to activate the
compiler-builtins `mem` feature which demangles important memory-related
intrinsics.