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/fiber/src/unix/aarch64.rs

@@ -18,6 +18,8 @@
 //   `DW_CFA_AARCH64_negate_ra_state` DWARF operation (aliased with the
 //   `.cfi_window_save` assembler directive) informs an unwinder about this
 
+use wasmtime_asm_macros::asm_func;
+
 cfg_if::cfg_if! {
     if #[cfg(target_os = "macos")] {
         macro_rules! cfi_window_save { () => (""); }

crates/fiber/src/unix/arm.rs

@@ -8,6 +8,8 @@
 // Also at this time this file is heavily based off the x86_64 file, so you'll
 // probably want to read that one as well.
 
+use wasmtime_asm_macros::{asm_func, asm_sym};
+
 // fn(top_of_stack(%r0): *mut u8)
 asm_func!(
     "wasmtime_fiber_switch",

crates/fiber/src/unix/x86.rs

@@ -10,6 +10,8 @@
 // different so the reserved space at the top of the stack is 8 bytes, not 16
 // bytes. Still two pointers though.
 
+use wasmtime_asm_macros::{asm_func, asm_sym};
+
 // fn(top_of_stack: *mut u8)
 asm_func!(
     "wasmtime_fiber_switch",

crates/fiber/src/unix/x86_64.rs

@@ -5,6 +5,8 @@
 // all the other bits. Documentation tries to reference various bits here and
 // there but try to make sure to read over everything before tweaking things!
 
+use wasmtime_asm_macros::{asm_func, asm_sym};
+
 // fn(top_of_stack(rdi): *mut u8)
 asm_func!(
     "wasmtime_fiber_switch",