feat: add loongarch64-unknown-linux-gnu image (#1465)

This PR introduces a basic build-only image for the
`loongarch64-unknown-linux-gnu` target. The Debian port for LoongArch is
currently underway, and we anticipate further developments in the near
future.

Close #1404

.changes/1465.json

@@ -0,0 +1,5 @@
+{
+    "type": "added",
+    "description": "add loongarch64 support",
+    "issues": [1404]
+}

.github/ISSUE_TEMPLATE/b_issue_report.yml

@@ -52,6 +52,7 @@ body:
         - i686-unknown-freebsd
         - i686-unknown-linux-gnu
         - i686-unknown-linux-musl
+        - loongarch64-unknown-linux-gnu
         - mips64el-unknown-linux-gnuabi64
         - mips64el-unknown-linux-muslabi64
         - mips64-unknown-linux-gnuabi64

README.md

@@ -224,6 +224,7 @@ terminate.
 | `i686-linux-android` [1]               | 9.0.8  | 9.0.8  | ✓   | 6.1.0 |   ✓    |
 | `i686-pc-windows-gnu`                  | N/A    | 9.4    | ✓   | N/A   |   ✓    |
 | `i686-unknown-linux-gnu`               | 2.31   | 9.4.0  | ✓   | 6.1.0 |   ✓    |
+| `loongarch64-unknown-linux-gnu`        | 2.36   | 13.2.0 | ✓   | N/A   |        |
 | `mips-unknown-linux-gnu`               | 2.30   | 9.4.0  | ✓   | 6.1.0 |   ✓    |
 | `mips-unknown-linux-musl`              | 1.2.3  | 9.2.0  | ✓   | 6.1.0 |   ✓    |
 | `mips64-unknown-linux-gnuabi64`        | 2.30   | 9.4.0  | ✓   | 6.1.0 |   ✓    |