cc: Pull out common library compilation flags

In preparation for building stubs from Rust, refactor how compilation
flags are calculated by pulling the common compilation flags out of
decorator-specific flags functions so they can be reused when building
stubs from Rust modules.

Bug: 203478530
Test: m blueprint_tests
Change-Id: If7a644ea3105d3fb8b2b5110b4f07ca053d1c5a3