Update unstable book with global_asm feature

Author: mrhota

src/doc/unstable-book/src/global_asm.md

@@ -0,0 +1,45 @@
+# `global_asm`
+
+The tracking issue for this feature is: [#35119]
+
+[#35119]: https://github.com/rust-lang/rust/issues/35119
+
+------------------------
+
+The `global_asm!` macro allows the programmer to write arbitrary
+assembly outside the scope of a function body, passing it through
+`rustc` and `llvm` to the assembler. The macro is a no-frills
+interface to LLVM's concept of module-level assembly. That is, all
+caveats, constraints, injunctions, rules, best practices, terms, and
+conditions apply to assembly written with `global_asm!`.
+
+`global_asm!` fills a role currently not satisfied by either `asm!`
+or `#[naked]` functions. With it, a programmer may define arbitrary
+symbols, assembler macros, functions, or _any_ other low-level
+behavior. The programmer has all features of the assembler at their
+disposal. This means linker will expect to resolve any symbols
+defined by the macro, modulo any symbols marked as external. It also
+means syntax for directives and assembly follow the conventions of
+the assembler in your toolchain.
+
+A simple usage looks like this:
+
+```rust,ignore
+global_asm!(r#"
+    .global my_asm_func
+    .hidden __cancel
+my_asm_func:
+    ret
+__other_cancel:
+    jmp __cancel
+"#);
+
+extern "C" {
+    fn my_asm_func();
+    fn __other_cancel();
+}
+
+fn __cancel() {
+    // do strange stuff
+}
+```