psm: allow manual opt out of #cfg[link("psm_s")]

Currenty, when using the `asm` configuration, the `lib.rs` file will manually
add a `link("psm_s")` attribute to the build causing a flag like `-lpsm_s`
to appear on the link line. `psm_s` is the library of assembly code, built by
`build.rs` in Cargo projects.

However, when using Cargo packages with build systems like Buck2, we have to
manually replace the `build.rs` script, and build the bits of C/assembly code
that come with `psm` and `stacker` as a separate build item (an actual library),
then link them into the Rust libraries.

The name of the library often can't be easily made identical to `psm_s` as
desired by the `link()` call, meaning that just blindly compiling this crate
causes an inevitable linking failure. But we're building the code and explicitly
linking the library anyway, removing the need for this `#[link]` clause.

Therefore, introduce a new `link_asm` cfg option to give "expert" users the
ability to link in code manually. This is always enabled by the Cargo build for
`asm`-compatible targets, but Buck users can leave it off.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

psm/build.rs

@@ -7,7 +7,7 @@ fn find_assembly(
  env: &str,
  masm: bool,
 ) -> Option<(&'static str, bool)> {
- println!("cargo::rustc-check-cfg=cfg(switchable_stack,asm)");
+ println!("cargo::rustc-check-cfg=cfg(switchable_stack,asm,link_asm)");
  match (arch, endian, os, env) {
  // The implementations for stack switching exist, but, officially, doing so without Fibers
  // is not supported in Windows. For x86_64 the implementation actually works locally,
@@ -79,6 +79,7 @@ fn main() {
 
  let asm = if let Some((asm, canswitch)) = find_assembly(&arch, &endian, &os, &env, masm) {
  println!("cargo:rustc-cfg=asm");
+ println!("cargo:rustc-cfg=link_asm");
  if canswitch {
  println!("cargo:rustc-cfg=switchable_stack")
  }

psm/src/lib.rs

@@ -56,7 +56,7 @@ macro_rules! extern_item {
 // NB: this could be nicer across multiple blocks but we cannot do it because of
 // https://github.com/rust-lang/rust/issues/65847
 extern_item! { {
- #![cfg_attr(asm, link(name="psm_s"))]
+ #![cfg_attr(link_asm, link(name="psm_s"))]
 
  #[cfg(asm)]
  fn rust_psm_stack_direction() -> u8;