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/Cargo.toml

@@ -18,6 +18,7 @@ cfg-if = "1.0"
 
 [target.'cfg(unix)'.dependencies]
 rustix = { version = "0.35.6", features = ["mm", "param"] }
+wasmtime-asm-macros = { version = "=0.40.0", path = "../asm-macros" }
 
 [target.'cfg(windows)'.dependencies.windows-sys]
 version = "0.36.1"