Auto merge of #131816 - Zalathar:profiler-feature, r=Kobzol

Make `profiler_builtins` an optional dependency of sysroot, not std

This avoids unnecessary rebuilds of std (and the compiler) when `build.profiler` is toggled off or on.

Fixes #131812.

---

Background: The `profiler_builtins` crate has been an optional dependency of std (behind a cargo feature) ever since it was added back in #42433. But as far as I can tell that has only ever been a convenient way to force the crate to be built, not a genuine dependency.

The side-effect of this false dependency is that toggling `build.profiler` causes a rebuild of std and the compiler, which shouldn't be necessary. This PR therefore makes `profiler_builtins` an optional dependency of the dummy sysroot crate (#108865), rather than a dependency of std.

What makes this change so small is that all of the necessary infrastructure already exists. Previously, bootstrap would enable the `profiler` feature on the sysroot crate, which would forward that feature to std. Now, enabling that feature directly enables sysroot's `profiler_builtins` dependency instead.

---

I believe this is more of a bootstrap change than a libs change, so tentatively:
r? bootstrap