doc/rustc: Move loongarch64-unknown-linux-musl to Tier 3

Fixes rust-lang#122266

(cherry picked from commit 1bad698)

Author: heiher

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

@@ -155,7 +155,6 @@ target | std | notes
 `i686-unknown-freebsd` | ✓ | 32-bit FreeBSD
 `i686-unknown-linux-musl` | ✓ | 32-bit Linux with MUSL
 [`i686-unknown-uefi`](platform-support/unknown-uefi.md) | * | 32-bit UEFI
-[`loongarch64-unknown-linux-musl`](platform-support/loongarch-linux.md) | ? |  | LoongArch64 Linux (LP64D ABI)
 `mips-unknown-linux-musl` | ✓ | MIPS Linux with MUSL
 `mips64-unknown-linux-muslabi64` | ✓ | MIPS64 Linux, n64 ABI, MUSL
 `mips64el-unknown-linux-muslabi64` | ✓ | MIPS64 (LE) Linux, n64 ABI, MUSL
@@ -269,6 +268,7 @@ target | std | host | notes
 `i686-uwp-windows-gnu` | ? |  |
 `i686-uwp-windows-msvc` | ? |  |
 `i686-wrs-vxworks` | ? |  |
+[`loongarch64-unknown-linux-musl`](platform-support/loongarch-linux.md) | ? |  | LoongArch64 Linux (LP64D ABI)
 [`loongarch64-unknown-none`](platform-support/loongarch-none.md) | * | LoongArch64 Bare-metal (LP64D ABI)
 [`loongarch64-unknown-none-softfloat`](platform-support/loongarch-none.md) | * | LoongArch64 Bare-metal (LP64S ABI)
 [`m68k-unknown-linux-gnu`](platform-support/m68k-unknown-linux-gnu.md) | ? |  | Motorola 680x0 Linux