add a fixme to use extern_custom when available

Author: folkertdev

library/compiler-builtins/compiler-builtins/src/probestack.rs

@@ -52,7 +52,8 @@
 // Our goal here is to touch each page between %rsp+8 and %rsp+8-%rax,
 // ensuring that if any pages are unmapped we'll make a page fault.
 //
-// This function is unsafe because it uses a custom ABI, it does not actually match `extern "C"`.
+// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
+// it does not actually match `extern "C"`.
 //
 // The ABI here is that the stack frame size is located in `%rax`. Upon
 // return we're not supposed to modify `%rsp` or `%rax`.
@@ -143,7 +144,8 @@ pub unsafe extern "C" fn __rust_probestack() {
 // that on Unix we're expected to restore everything as it was, this
 // function basically can't tamper with anything.
 //
-// This function is unsafe because it uses a custom ABI, it does not actually match `extern "C"`.
+// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
+// it does not actually match `extern "C"`.
 //
 // The ABI here is the same as x86_64, except everything is 32-bits large.
 #[unsafe(naked)]
@@ -190,7 +192,8 @@ pub unsafe extern "C" fn __rust_probestack() {
 // probestack function will also do things like _chkstk in MSVC.
 // So we need to sub %ax %sp in probestack when arch is x86.
 //
-// This function is unsafe because it uses a custom ABI, it does not actually match `extern "C"`.
+// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
+// it does not actually match `extern "C"`.
 //
 // REF: Rust commit(74e80468347)
 // rust\src\llvm-project\llvm\lib\Target\X86\X86FrameLowering.cpp: 805