Clean up `library/profiler_builtins/build.rs`
This PR makes a series of improvements to the long-neglected build script for `profiler_builtins`.
Most notably:
- The logic that silently skips missing source files has been removed, since it is currently unnecessary and makes build errors more confusing.
- The script now emits `cargo::rerun-if-changed` directives for the `compiler-rt` source and include directories.
Compiler behaviour and user programs should be unaffected by these changes.
6a96e7a255