rust: pin-init: add `std` and `alloc` support from the user-space version

rust: pin-init: add `std` and `alloc` support from the user-space version

To synchronize the kernel's version of pin-init with the user-space
version, introduce support for `std` and `alloc`. While the kernel uses
neither, the user-space version has to support both. Thus include the
required `#[cfg]`s and additional code.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-17-benno.lossin@proton.me
[ Undo the temporary `--extern force:alloc` since now we have contents
  for `alloc` here. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>