doc: Add *-unknown-managarm-mlibc documentation

src/doc/rustc/src/SUMMARY.md

@@ -58,6 +58,7 @@
     - [\*-nto-qnx-\*](platform-support/nto-qnx.md)
     - [*-unikraft-linux-musl](platform-support/unikraft-linux-musl.md)
     - [*-unknown-hermit](platform-support/hermit.md)
+    - [\*-unknown-managarm-mlibc](platform-support/managarm.md)
     - [\*-unknown-netbsd\*](platform-support/netbsd.md)
     - [*-unknown-openbsd](platform-support/openbsd.md)
     - [\*-unknown-uefi](platform-support/unknown-uefi.md)

src/doc/rustc/src/platform-support.md

@@ -373,6 +373,7 @@ target | std | host | notes
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku
 [`x86_64-unknown-hermit`](platform-support/hermit.md) | ✓ |  | x86_64 Hermit
 `x86_64-unknown-l4re-uclibc` | ? |  |
+[`x86_64-unknown-managarm-mlibc`](platform-support/managarm.md) |   |   | x86_64 managarm
 [`x86_64-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | 64-bit OpenBSD
 `x86_64-uwp-windows-gnu` | ✓ |  |
 `x86_64-uwp-windows-msvc` | ✓ |  |

src/doc/rustc/src/platform-support/managarm.md

@@ -0,0 +1,63 @@
+# `*-unknown-managarm-mlibc`
+
+**Tier: 3**
+
+## Target Maintainers
+
+- [@no92](https://github.com/no92)
+- [@64](https://github.com/64)
+- [@Dennisbonke](https://github.com/Dennisbonke)
+
+## Requirements
+
+This target is cross-compiled. There is currently no support for `std` yet. It generates binaries in the ELF format.
+
+## Building the target
+
+For now, building a patched LLVM with [our patches located here](https://github.com/managarm/bootstrap-managarm/tree/master/patches/llvm) is necessary.
+
+Once that is done, set up your `config.toml` like this:
+
+```toml
+change-id = 102579
+
+[llvm]
+targets = "X86"
+download-ci-llvm = false
+
+[build]
+target = ["x86_64-unknown-managarm-mlibc", "x86_64-unknown-linux-gnu"]
+build-dir = "/path/to/the/build/dir"
+docs = false
+
+[install]
+prefix = "/path/to/the/prefix"
+sysconfdir = "etc"
+
+[rust]
+codegen-tests = false
+deny-warnings = false
+
+[target.x86_64-unknown-linux-gnu]
+llvm-config = "/path/to/your/llvm/bin/llvm-config"
+
+[target.x86_64-unknown-managarm-mlibc]
+llvm-config = "/path/to/your/llvm/bin/llvm-config"
+```
+
+## Building Rust programs
+
+Build a `x86_64-managarm-gcc` using our [gcc fork](https://github.com/managarm/gcc).
+
+```toml
+[build]
+rustc = "/path/to/the/rust-prefix/bin/rustc"
+target = "x86_64-unknown-managarm-mlibc"
+
+[target.x86_64-unknown-managarm-mlibc]
+linker = "/path/to/the/managarm-gcc/bin/x86_64-managarm-gcc"
+```
+
+## Testing
+
+This target does not support running the Rust testsuite yet.