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/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+authors = ["The Wasmtime Project Developers"]
+description = "Macros for defining asm functions in Wasmtime"
+edition = "2021"
+license = "Apache-2.0 WITH LLVM-exception"
+name = "wasmtime-asm-macros"
+repository = "https://github.com/bytecodealliance/wasmtime"
+version = "0.40.0"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+cfg-if = "1"