rustbuild: Move compiler-builtins build logic to manifest

This commit moves the compiler-builtins-specific build logic from
`src/bootstrap/bin/rustc.rs` into the workspace `Cargo.toml`'s
`[profile]` configuration. Now that rust-lang/cargo#7253 is fixed we can
ensure that Cargo knows about debug assertions settings, and it can also
be configured to specifically disable debug assertions unconditionally
for compiler-builtins. This should improve rebuild logic when
debug-assertions settings change and also improve build-std integration
where Cargo externally now has an avenue to learn how to build
compiler-builtins as well.

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