Rollup merge of #153936 - danielzgtg:perf/immediateAbortAvoidPthreadGetattrNp, r=Mark-Simulacrum

Skip stack_start_aligned for immediate-abort

This improves startup performance by 16%, shown by an optimized hello-world program. glibc's `pthread_getattr_np` performs expensive syscalls when reading `/proc/self/maps`. That is all wasted with `panic = immediate-abort` active because `init()` immediately discards the return value from `install_main_guard()`. A similar improvement can be seen in environments that don't have `/proc`. This change is safe because the immediately succeeding comment says that we rely on Linux's "own stack-guard mechanism".

Tracking issue: https://github.com/rust-lang/rust/issues/147286

# Benchmark

Set it up with `cargo new hello-world2`, and replace these files:

```toml
# Cargo.toml
cargo-features = ["panic-immediate-abort"]

[package]
name = "hello-world"
version = "0.1.0"
edition = "2024"

[profile.release]
lto = true
panic = "immediate-abort"
codegen-units = 1
opt-level = "z"
strip = true

# .cargo/config.toml
[unstable]
build-std = ["std"]
```

## Before

```console
home@daniel-desktop3:~/CLionProjects/hello-world2$ hyperfine -N target/release/hello-world2
Benchmark 1: target/release/hello-world2
  Time (mean ± σ):     524.8 µs ±  65.1 µs    [User: 276.1 µs, System: 187.0 µs]
  Range (min … max):   446.4 µs … 975.5 µs    3996 runs

home@daniel-desktop3:~/CLionProjects/hello-world2$ hyperfine -N target/release/hello-world2
Benchmark 1: target/release/hello-world2
  Time (mean ± σ):     519.4 µs ±  65.8 µs    [User: 282.1 µs, System: 177.7 µs]
  Range (min … max):   443.2 µs … 830.5 µs    3612 runs

home@daniel-desktop3:~/CLionProjects/hello-world2$ hyperfine -N target/release/hello-world2
Benchmark 1: target/release/hello-world2
  Time (mean ± σ):     520.0 µs ±  64.3 µs    [User: 277.1 µs, System: 182.1 µs]
  Range (min … max):   447.1 µs … 1001.3 µs    3804 runs
```

For a visualization of the problem, run `cargo +stage1 build --release && perf record --call-graph dwarf -F max ./target/release/hello-world2 && perf script | inferno-collapse-perf | inferno-flamegraph > flamegraph.svg`:

<img width="3832" height="1216" alt="flamegraph with 17.41% __pthread_getattr_np" src="https://github.com/user-attachments/assets/acc2286e-1582-4772-9e3b-68b5c35e3e70" />

## After

```console
home@daniel-desktop3:~/CLionProjects/hello-world2$ hyperfine -N target/release/hello-world2Benchmark 1: target/release/hello-world2
  Time (mean ± σ):     444.7 µs ±  57.3 µs    [User: 257.4 µs, System: 130.2 µs]
  Range (min … max):   379.4 µs … 1289.3 µs    3893 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.

home@daniel-desktop3:~/CLionProjects/hello-world2$ hyperfine -N target/release/hello-world2
Benchmark 1: target/release/hello-world2
  Time (mean ± σ):     452.3 µs ±  60.7 µs    [User: 261.5 µs, System: 133.5 µs]
  Range (min … max):   374.9 µs … 1512.4 µs    4177 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.

home@daniel-desktop3:~/CLionProjects/hello-world2$ hyperfine -N target/release/hello-world2
Benchmark 1: target/release/hello-world2
  Time (mean ± σ):     441.2 µs ±  56.1 µs    [User: 256.2 µs, System: 128.8 µs]
  Range (min … max):   375.0 µs … 760.4 µs    4032 runs
```