Add a platform doc for hexagon-unknown-linux-musl

Author: androm3da

src/doc/rustc/src/platform-support/hexagon-unknown-linux-musl.md

@@ -0,0 +1,89 @@
+# `hexagon-unknown-linux-musl`
+
+**Tier: 3**
+
+Target for cross-compiling Linux user-mode applications targeting the Hexagon
+DSP architecture.
+
+| Target                   | Descriptions                              |
+| ------------------------ | ----------------------------------------- |
+| hexagon-unknown-linux-musl | Hexagon 32-bit Linux |
+
+## Target maintainers
+
+- [Brian Cain](https://github.com/androm3da), `bcain@quicinc.com`
+
+## Requirements
+The target is cross-compiled. This target supports `std`.  By default, code
+generated with this target should run on Hexagon DSP hardware.
+
+- `-Ctarget-cpu=hexagonv73` adds support for instructions defined up to Hexagon V73.
+
+Binaries can be run using QEMU user emulation. On Debian-based systems, it should be
+sufficient to install the package `qemu-user-static` to be able to run simple static
+binaries:
+
+```text
+# apt install qemu-user-static
+# qemu-hexagon ./hello
+```
+
+In order to build linux programs with Rust, you will require a linker capable
+of targeting hexagon.  You can use `clang`/`lld` from the [hexagon toolchain
+using exclusively public open source repos](https://github.com/quic/toolchain_for_hexagon/releases).
+
+Also included in that toolchain is the C library that can be used when creating
+dynamically linked executables.
+
+```text
+# ./clang+llvm-99.9-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/bin/qemu-hexagon -L ./clang+llvm-99.9-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr/ ./hello
+```
+
+## Building the target
+Because it is Tier 3, rust does not yet ship pre-compiled artifacts for this target.
+
+Therefore, you can build Rust with support for the target by adding it to the
+target list in config.toml, a sample configuration is shown below.
+
+```toml
+[build]
+target = [ "hexagon-unknown-linux-musl"]
+docs = false
+
+[target.hexagon-unknown-linux-musl]
+
+cc = "hexagon-unknown-linux-musl-clang"
+cxx = "hexagon-unknown-linux-musl-clang++"
+linker = "hexagon-unknown-linux-musl-clang"
+ar = "hexagon-unknown-linux-musl-ar"
+ranlib = "hexagon-unknown-linux-musl-ranlib"
+musl-root = "/usr/local/clang+llvm-18.0.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr"
+llvm-libunwind = 'in-tree'
+qemu-rootfs = "/usr/local/clang+llvm-18.0.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr"
+```
+
+
+## Testing
+
+Currently there is no support to run the rustc test suite for this target.
+
+
+```text
+rustc --target hexagon-unknown-linux-musl your-code.rs
+```
+
+## Building Rust programs
+
+The following `.cargo/config` is needed inside any project directory to build
+for the Hexagon Linux target:
+
+```toml
+[build]
+target = "hexagon-unknown-linux-musl"
+
+[target.hexagon-unknown-linux-musl]
+linker = "hexagon-unknown-linux-musl-clang"
+ar = "hexagon-unknown-linux-musl-ar"
+runner = "/usr/bin/env QEMU_LD_PREFIX=/usr/local/clang+llvm-18.0.0-rc1-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr qemu-hexagon"
+```
+