rust: kbuild: deduplicate pin-init flags

rust: kbuild: deduplicate pin-init flags

Extract `pin_init{,_internal}-{cfgs,flags}` to reduce duplication.

[ The new variables will be used later on to easily pass them to
  the `scripts/generate_rust_analyzer.py` script. - Miguel ]

Acked-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260127-rust-analyzer-pin-init-duplication-v3-1-118c48c35e88@kernel.org
[ Rebased. Moved new variables above. Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>