dev-lang/rust: add backported version of pr65474

pr65474 minimizes the amount of files needed for the install
of a rust-std component by not merging certain dev-header
files and other instable components.

more information:
rust-lang/rust#64823
rust-lang/rust#65474

Signed-off-by: Steffen Kuhn <[email protected]>

Author: stefson