docs: update Motor OS target docs

Update the docs to reflect that
[Motor OS std library PR](#147000)
has been merged.

Author: lasiotus

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

@@ -432,7 +432,7 @@ target | std | host | notes
 `x86_64-unknown-l4re-uclibc` | ? |  |
 [`x86_64-unknown-linux-none`](platform-support/x86_64-unknown-linux-none.md) | * |  | 64-bit Linux with no libc
 [`x86_64-unknown-managarm-mlibc`](platform-support/managarm.md) | ? |   | x86_64 Managarm
-[`x86_64-unknown-motor`](platform-support/motor.md) | ? |  | x86_64 Motor OS
+[`x86_64-unknown-motor`](platform-support/motor.md) | ✓ |  | x86_64 Motor OS
 [`x86_64-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | 64-bit OpenBSD
 [`x86_64-unknown-trusty`](platform-support/trusty.md) | ✓ |  |
 `x86_64-uwp-windows-gnu` | ✓ |  |

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

@@ -15,27 +15,53 @@ This target is cross-compiled. There are no special requirements for the host.
 
 Motor OS uses the ELF file format.
 
-## Building the target
+## Building the target toolchain
 
-The target can be built by enabling it for a `rustc` build, for example:
+Motor OS target toolchain can be
+[built using `x.py`](https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html):
+
+The bootstrap file:
 
 ```toml
+# bootstrap.toml
+#
+# Place it in the root of rust-lang/rust repo.
+
+change-id = 146458
+
+profile = "library"
+
 [build]
-build-stage = 2
-target = ["x86_64-unknown-motor"]
+host = ["x86_64-unknown-linux-gnu"]
+target = ["x86_64-unknown-linux-gnu", "x86_64-unknown-motor"]
+
+[rust]
+deny-warnings = false
+incremental = true
+```
+
+The build command:
+
+```sh
+$ cd $MOTORH/rust
+$ TARGET=x86_64-unknown-motor \
+    CARGO_CFG_TARGET_ARCH=x86_64 \
+    CARGO_CFG_TARGET_VENDOR=unknown \
+    CARGO_CFG_TARGET_OS=motor \
+    CARGO_CFG_TARGET_ENV="" \
+    ./x.py build --stage 2 clippy library
 ```
 
 ## Building Rust programs
 
-Rust standard library is fully supported/implemented, but is not yet part of
-the official Rust repo, so an out-of-tree building process should be
-followed, as described in the
-[build doc](https://github.com/moturus/motor-os/blob/main/docs/build.md).
+See the [Hello Motor OS](https://github.com/moturus/motor-os/blob/main/docs/recipes/hello-motor-os.md)
+example.
 
 ## Testing
 
 Cross-compiled Rust binaries and test artifacts can be executed in Motor OS VMs,
-as described in e.g.
+as described in the [build doc](https://github.com/moturus/motor-os/blob/main/docs/build.md)
+and the
 [Hello Motor OS](https://github.com/moturus/motor-os/blob/main/docs/recipes/hello-motor-os.md)
 example.