Split global_asm! helper macros out from wasmtime-fibers (#4454)

This moves them into a new `wasmtime-asm-macros` crate that can be used not just
from the `wasmtime-fibers` crate but also from other crates (e.g. we will need
them in https://github.com/bytecodealliance/wasmtime/pull/4431).

crates/asm-macros/src/lib.rs

@@ -0,0 +1,66 @@
+//! This crate defines a macro named `asm_func!` which is suitable for
+//! generating a single `global_asm!`-defined function.
+//!
+//! This macro takes care of platform-specific directives to get the symbol
+//! attributes correct (e.g. ELF symbols get a size and are flagged as a
+//! function) and additionally handles visibility across platforms. All symbols
+//! should be visible to Rust but not visible externally outside of a `*.so`.
+
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "macos")] {
+        #[macro_export]
+        macro_rules! asm_func {
+            ($name:tt, $($body:tt)*) => {
+                std::arch::global_asm!(concat!(
+                    ".p2align 4\n",
+                    ".private_extern _", $name, "\n",
+                    ".global _", $name, "\n",
+                    "_", $name, ":\n",
+                    $($body)*
+                ));
+            };
+        }
+
+        #[macro_export]
+        macro_rules! asm_sym {
+            ($name:tt) => (concat!("_", $name))
+        }
+    } else {
+        // Note that for now this "else" clause just assumes that everything
+        // other than macOS is ELF and has the various directives here for
+        // that.
+        cfg_if::cfg_if! {
+            if #[cfg(target_arch = "arm")] {
+                #[macro_export]
+                macro_rules! elf_func_type_header {
+                    ($name:tt) => (concat!(".type ", $name, ",%function\n"))
+                }
+            } else {
+                #[macro_export]
+                macro_rules! elf_func_type_header {
+                    ($name:tt) => (concat!(".type ", $name, ",@function\n"))
+                }
+            }
+        }
+
+        #[macro_export]
+        macro_rules! asm_func {
+            ($name:tt, $($body:tt)*) => {
+                std::arch::global_asm!(concat!(
+                    ".p2align 4\n",
+                    ".hidden ", $name, "\n",
+                    ".global ", $name, "\n",
+                    $crate::elf_func_type_header!($name),
+                    $name, ":\n",
+                    $($body)*
+                    ".size ", $name, ",.-", $name,
+                ));
+            };
+        }
+
+        #[macro_export]
+        macro_rules! asm_sym {
+            ($name:tt) => ($name)
+        }
+    }
+}