doc/rustc: promote aarch64-unknown-linux-gnu to tier 1

This also adds a note about missing stack probes support, per the
discussion on RFC 2959.

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

@@ -34,6 +34,7 @@ Specifically they will each satisfy the following requirements:
 
 target | std | host | notes
 -------|-----|------|-------
+`aarch64-unknown-linux-gnu` | ✓ | ✓ | ARM64 Linux (kernel 4.2, glibc 2.17+) [^missing-stack-probes]
 `i686-pc-windows-gnu` | ✓ | ✓ | 32-bit MinGW (Windows 7+)
 `i686-pc-windows-msvc` | ✓ | ✓ | 32-bit MSVC (Windows 7+)
 `i686-unknown-linux-gnu` | ✓ | ✓ | 32-bit Linux (kernel 2.6.32+, glibc 2.11+)
@@ -42,6 +43,12 @@ target | std | host | notes
 `x86_64-pc-windows-msvc` | ✓ | ✓ | 64-bit MSVC (Windows 7+)
 `x86_64-unknown-linux-gnu` | ✓ | ✓ | 64-bit Linux (kernel 2.6.32+, glibc 2.11+)
 
+[^missing-stack-probes]: Stack probes support is missing on
+  `aarch64-unknown-linux-gnu`, but it's planned to be implemented in the near
+  future. The implementation is tracked on [issue #77071][77071].
+
+[77071]: https://github.com/rust-lang/rust/issues/77071
+
 ## Tier 2
 
 Tier 2 platforms can be thought of as "guaranteed to build". Automated tests
@@ -62,7 +69,6 @@ target | std | host | notes
 `aarch64-fuchsia` | ✓ |  | ARM64 Fuchsia
 `aarch64-linux-android` | ✓ |  | ARM64 Android
 `aarch64-pc-windows-msvc` | ✓ | ✓ | ARM64 Windows MSVC
-`aarch64-unknown-linux-gnu` | ✓ | ✓ | ARM64 Linux (kernel 4.2, glibc 2.17)
 `aarch64-unknown-linux-musl` | ✓ | ✓ | ARM64 Linux with MUSL
 `aarch64-unknown-none` | * |  | Bare ARM64, hardfloat
 `aarch64-unknown-none-softfloat` | * |  | Bare ARM64, softfloat