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/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",