add -Zexport-executable-symbols to unstable book

src/doc/unstable-book/src/compiler-flags/export-executable-symbols.md

@@ -0,0 +1,43 @@
+# `export-executable-symbols`
+
+The tracking issue for this feature is: [#84161](https://github.com/rust-lang/rust/issues/84161).
+
+------------------------
+
+The `-Zexport-executable-symbols` compiler flag makes `rustc` export symbols from executables. The resulting binary is runnable, but can also be used as a dynamic library. This is useful for interoperating with programs written in other languages, in particular languages with a runtime like Java or Lua.
+
+For example on windows:
+```rust
+#[no_mangle]
+fn my_function() -> usize {
+    return 42;
+}
+
+fn main() {
+    println!("Hello, world!");
+}
+```
+
+A standard `cargo build` will produce a `.exe` without an export directory. When the `export-executable-symbols` flag is added
+
+```Bash
+export RUSTFLAGS="-Zexport-executable-symbols"
+cargo build
+```
+
+the binary has an export directory with the functions:
+
+```plain
+The Export Tables (interpreted .edata section contents)
+
+...
+
+[Ordinal/Name Pointer] Table
+    [   0] my_function
+    [   1] main
+```
+(the output of `objdump -x` on the binary)
+
+Please note that the `#[no_mangle]` attribute is required. Without it, the symbol is not exported.
+
+The equivalent of this flag in C and C++ compilers is the `__declspec(dllexport)` annotation or the `-rdynamic` linker flag.