Temporarily remove toolchain/ from androidmk_denylist.go

There are defensive `Android.mk` files present in several top-level
toolchain/ projects. Enforcing the denylist strictly breaks several
builds, so this change temporarily relaxes that by removing our
directories from the list.

Test: Run Rust's test_compiler.py
Change-Id: Ic00a305b78485543d098aeea42c2d2b93f979430