Profile-Guided Optimization (PGO) benchmarks

Hi!

I tried to apply Profile-Guided Optimization (PGO) to optimize `llrt` performance further (as I already did for many other projects - see all current results [here](https://github.com/zamazan4ik/awesome-pgo)). I performed some basic benchmarks and want to share the results here.

## Test environment

* Fedora 39
* Linux kernel 6.7.3
* AMD Ryzen 9 5900x
* 48 Gib RAM
* SSD Samsung 980 Pro 2 Tib
* Compiler - Rustc 1.76
* llrt version: the latest for now from the `main` branch on commit `c040bfd05a2be8d3300e7a1bbfc9405c42a865fa`
* Disabled Turbo boost (for more stable results across benchmark runs)

## Benchmark

As a benchmark, I use the same command as I found in the Makefile - `llrt fixtures/hello.js`. The same scenario is used for the PGO training phase. All PGO optimization steps are done with [cargo-pgo](https://github.com/Kobzol/cargo-pgo) tool. PGO instrumented version is built with `cargo pgo build`, PGO optimized version - `cargo pgo optimize build`. `taskset -c 0` is used for reducing CPU scheduling influence on the results.

## Results

I got the following results:
```
hyperfine -u microsecond -N --warmup=2000 --min-runs 10000 "taskset -c 0 ./llrt_optimized ../fixtures/hello.js" "taskset -c 0 ./llrt_release ../fixtures/hello.js"
Benchmark 1: taskset -c 0 ./llrt_optimized ../fixtures/hello.js
Time (mean ± σ): 2664.8 µs ± 78.8 µs [User: 590.1 µs, System: 1943.3 µs]
Range (min … max): 2478.1 µs … 4486.1 µs 10000 runs

Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.

Benchmark 2: taskset -c 0 ./llrt_release ../fixtures/hello.js
Time (mean ± σ): 2796.1 µs ± 63.6 µs [User: 601.4 µs, System: 2068.9 µs]
Range (min … max): 2647.5 µs … 4495.0 µs 10000 runs

Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.

Summary
taskset -c 0 ./llrt_optimized ../fixtures/hello.js ran
1.05 ± 0.04 times faster than taskset -c 0 ./llrt_release ../fixtures/hello.js
```
, where `llrt_release` - usual Release version, `llrt_optimized` - PGO-optimized version.

I ran the benchmark multiple times, with different command orders, etc - in all cases, the PGO-optimized version was faster than the usual release version. However, it would be awesome to perform some more precise benchmarks.

## Further steps

I can suggest to do the following things:

* Perform more PGO benchmarks with some more precise performance measurements.
* If PGO is worth it - add a note to the documentation about it and, possibly, make an option in the build scripts to optimize llrt easier with the existing build infrastructure.
* Try to play with Post-Link Optimization (PLO) with tools like [LLVM BOLT](https://github.com/llvm/llvm-project/blob/main/bolt/README.md).

I hope these benchmark results can be interesting to someone.

Start with the Makefile command `llrt fixtures/hello.js` and the benchmark setup using cargo-pgo, hyperfine, and taskset. Reproduce the reported PGO comparison on the stated fixture, then determine whether more precise measurements justify a documentation note or build-script option; the issue does not name a specific file or test.

Written by the indexing model from the issue text.