update uclibc instructions for new toolchain, add link from platforms doc

Author: skrap

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

@@ -220,7 +220,7 @@ target | std | host | notes
 `armv6-unknown-netbsd-eabihf` | ? |  |
 `armv6k-nintendo-3ds` | * |  | ARMv6K Nintendo 3DS, Horizon (Requires devkitARM toolchain)
 `armv7-apple-ios` | ✓ |  | ARMv7 iOS, Cortex-a8
-`armv7-unknown-linux-uclibceabihf` | ✓ | ? | ARMv7 Linux uClibc
+[`armv7-unknown-linux-uclibceabihf`](platform-support/armv7-unknown-linux-uclibceabihf.md) | ✓ | ? | ARMv7 Linux uClibc
 `armv7-unknown-freebsd` | ✓ | ✓ | ARMv7 FreeBSD
 `armv7-unknown-netbsd-eabihf` | ✓ | ✓ |
 `armv7-wrs-vxworks-eabihf` | ? |  |

src/doc/rustc/src/platform-support/armv7-unknown-linux-uclibceabihf.md

@@ -18,7 +18,7 @@ This target is cross compiled, and requires a cross toolchain.  You can find sui
 
 Compiling rust for this target has been tested on `x86_64` linux hosts.  Other host types have not been tested, but may work, if you can find a suitable cross compilation toolchain for them.
 
-If you don't already have a suitable toolchain, download one [here](https://toolchains.bootlin.com/downloads/releases/toolchains/armv7-eabihf/tarballs/armv7-eabihf--uclibc--bleeding-edge-2020.08-1.tar.bz2), and unpack it into a directory.
+If you don't already have a suitable toolchain, download one [here](https://toolchains.bootlin.com/downloads/releases/toolchains/armv7-eabihf/tarballs/armv7-eabihf--uclibc--bleeding-edge-2021.11-1.tar.bz2), and unpack it into a directory.
 
 ### Configure rust