Add platform support document links to tier table

Author: Michcioperz

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

@@ -244,9 +244,9 @@ target | std | host | notes
 `i686-uwp-windows-gnu` | ? |  |
 `i686-uwp-windows-msvc` | ? |  |
 `i686-wrs-vxworks` | ? |  |
-`m68k-unknown-linux-gnu` | ? |  | Motorola 680x0 Linux
+[`m68k-unknown-linux-gnu`](platform-support/m68k-unknown-linux-gnu.md) | ? |  | Motorola 680x0 Linux
 `mips-unknown-linux-uclibc` | ✓ |  | MIPS Linux with uClibc
-`mips64-openwrt-linux-musl` | ? |  | MIPS64 for OpenWrt Linux MUSL
+[`mips64-openwrt-linux-musl`](platform-support/mips64-openwrt-linux-musl.md) | ? |  | MIPS64 for OpenWrt Linux MUSL
 `mipsel-sony-psp` | * |  | MIPS (LE) Sony PlayStation Portable (PSP)
 `mipsel-unknown-linux-uclibc` | ✓ |  | MIPS (LE) Linux with uClibc
 `mipsel-unknown-none` | * |  | Bare MIPS (LE) softfloat