describe unstable linker flavors in the unstable book

src/doc/unstable-book/src/compiler-flags/codegen-options.md

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+# Unstable codegen options
+
+All of these options are passed to `rustc` via the `-C` flag, short for "codegen". The flags are
+stable but some of their values are individually unstable, and also require using `-Z
+unstable-options` to be accepted.
+
+## linker-flavor
+
+In addition to the stable set of linker flavors, the following unstable values also exist:
+- `ptx`: use [`rust-ptx-linker`](https://github.com/denzp/rust-ptx-linker)
+  for Nvidia NVPTX GPGPU support.
+- `bpf`: use [`bpf-linker`](https://github.com/alessandrod/bpf-linker) for eBPF support.
+
+Additionally, a set of more precise linker flavors also exists, for example allowing targets to
+declare that they use the LLD linker by default. The following values are currently unstable, and
+the goal is for them to become stable, and preferred in practice over the existing stable values:
+- `gnu`: unix-like linker with GNU extensions
+- `gnu-lld`: `gnu` using LLD
+- `gnu-cc`: `gnu` using a C/C++ compiler as the linker driver
+- `gnu-lld-cc`: `gnu` using LLD and a C/C++ compiler as the linker driver
+- `darwin`: unix-like linker for Apple targets
+- `darwin-lld`: `darwin` using LLD
+- `darwin-cc`: `darwin` using a C/C++ compiler as the linker driver
+- `darwin-lld-cc`: `darwin` using LLD and a C/C++ compiler as the linker driver
+- `wasm-lld`: unix-like linker for Wasm targets, with LLD
+- `wasm-lld-cc`: unix-like linker for Wasm targets, with LLD and a C/C++ compiler as the linker
+  driver
+- `unix`: basic unix-like linker for "any other Unix" targets (Solaris/illumos, L4Re, MSP430, etc),
+  not supported with LLD.
+- `unix-cc`: `unix` using a C/C++ compiler as the linker driver
+- `msvc-lld`: MSVC-style linker for Windows and UEFI, with LLD
+- `em-cc`: emscripten compiler frontend, similar to `wasm-lld-cc` with a different interface