Auto merge of #121101 - GnomedDev:dyn-small-c-string, r=Nilstrieb

Reduce monomorphisation bloat in small_c_string

This is a code path usually next to an FFI call, so taking the `dyn` slowdown for the 1159 llvm-line (fat lto, codegen-units 1, release build) drop in my testing program [t2fanrd](https://github.com/GnomedDev/t2fanrd) is worth it imo.

Author: bors

tests/fail/shims/fs/isolated_file.stderr

@@ -11,8 +11,9 @@ LL |         let fd = cvt_r(|| unsafe { open64(path.as_ptr(), flags, opts.mode a
    = note: inside `std::sys::pal::PLATFORM::cvt_r::<i32, {closure@std::sys::pal::PLATFORM::fs::File::open_c::{closure#0}}>` at RUSTLIB/std/src/sys/pal/PLATFORM/mod.rs:LL:CC
    = note: inside `std::sys::pal::PLATFORM::fs::File::open_c` at RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC
    = note: inside closure at RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC
-   = note: inside `std::sys::pal::PLATFORM::small_c_string::run_with_cstr::<std::sys::pal::PLATFORM::fs::File, {closure@std::sys::pal::PLATFORM::fs::File::open::{closure#0}}>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
-   = note: inside `std::sys::pal::PLATFORM::small_c_string::run_path_with_cstr::<std::sys::pal::PLATFORM::fs::File, {closure@std::sys::pal::PLATFORM::fs::File::open::{closure#0}}>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
+   = note: inside `std::sys::pal::PLATFORM::small_c_string::run_with_cstr_stack::<std::sys::pal::PLATFORM::fs::File>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
+   = note: inside `std::sys::pal::PLATFORM::small_c_string::run_with_cstr::<std::sys::pal::PLATFORM::fs::File>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
+   = note: inside `std::sys::pal::PLATFORM::small_c_string::run_path_with_cstr::<std::sys::pal::PLATFORM::fs::File>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
    = note: inside `std::sys::pal::PLATFORM::fs::File::open` at RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC
    = note: inside `std::fs::OpenOptions::_open` at RUSTLIB/std/src/fs.rs:LL:CC
    = note: inside `std::fs::OpenOptions::open::<&std::path::Path>` at RUSTLIB/std/src/fs.rs:LL:CC