Auto merge of #54592 - GabrielMajeri:no-plt, r=nagisa

Support for disabling PLT for better function call performance

This PR gives `rustc` the ability to skip the PLT when generating function calls into shared libraries. This can improve performance by reducing branch indirection.

AFAIK, the only advantage of using the PLT is to allow for ELF lazy binding. However, since Rust already [enables full relro for security](https://github.com/rust-lang/rust/pull/43170), lazy binding was disabled anyway.

This is a little known feature which is supported by [GCC](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html) and [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fplt) as `-fno-plt` (some Linux distros [enable it by default](https://git.archlinux.org/svntogit/packages.git/tree/trunk/makepkg.conf?h=packages/pacman#n40) for all builds).

Implementation inspired by [this patch](https://reviews.llvm.org/D39079#change-YvkpNDlMs_LT) which adds `-fno-plt` support to Clang.

## Performance

I didn't run a lot of benchmarks, but these are the results on my machine for a `clap` [benchmark](https://github.com/clap-rs/clap/blob/master/benches/05_ripgrep.rs):

```
 name              control ns/iter  no-plt ns/iter  diff ns/iter  diff %  speedup
 build_app_long    11,097           10,733                  -364  -3.28%   x 1.03
 build_app_short   11,089           10,742                  -347  -3.13%   x 1.03
 build_help_long   186,835          182,713               -4,122  -2.21%   x 1.02
 build_help_short  80,949           78,455                -2,494  -3.08%   x 1.03
 parse_clean       12,385           12,044                  -341  -2.75%   x 1.03
 parse_complex     19,438           19,017                  -421  -2.17%   x 1.02
 parse_lots        431,493          421,421              -10,072  -2.33%   x 1.02
```

A small performance improvement across the board, with no downsides. It's likely binaries which make a lot of function calls into dynamic libraries could see even more improvements. [This comment](https://patchwork.ozlabs.org/patch/468993/#1028255) suggests that, in some cases, `-fno-plt` could improve PIC/PIE code performance by 10%.

## Security benefits

**Bonus**: some of the speculative execution attacks rely on the PLT, by disabling it we reduce a big attack surface and reduce the need for [`retpoline`](https://reviews.llvm.org/D41723).

## Remaining PLT calls

The compiled binaries still have plenty of PLT calls, coming from C/C++ libraries. Building dependencies with `CFLAGS=-fno-plt CXXFLAGS=-fno-plt` removes them.

src/librustc_codegen_llvm/attributes.rs

@@ -137,6 +137,15 @@ pub fn apply_target_cpu_attr(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
             target_cpu.as_c_str());
 }
 
+/// Sets the `NonLazyBind` LLVM attribute on a given function,
+/// assuming the codegen options allow skipping the PLT.
+pub fn non_lazy_bind(sess: &Session, llfn: &'ll Value) {
+    // Don't generate calls through PLT if it's not necessary
+    if !sess.needs_plt() {
+        Attribute::NonLazyBind.apply_llfn(Function, llfn);
+    }
+}
+
 /// Composite function which sets LLVM attributes for function depending on its AST (#[attribute])
 /// attributes.
 pub fn from_fn_attrs(