feat: add loong64 (LoongArch) build support

Add loong64 (LoongArch) architecture support to the build system and CI:

- Makefile.linux: add loong64 to architectures that don't use -buildmode=pie
  (consistent with other non-amd64 architectures like mips, ppc64)
- ci.yml: add linux/loong64 to crossbuild matrix with CGO_ENABLED=0
- RELEASES.md: add linux/loong64 as Tier 3 (Build-verified) platform

The linux/loong64 release build and nightly build entries are intentionally
excluded — the cross-compilation toolchain is not yet available in Ubuntu 22.04
apt repositories (no crossbuild-essential-loong64 package). The crossbuild CI
check uses CGO_ENABLED=0. Nightly and release builds will be re-enabled once
the upstream tonistiigi/xx base image provides the loong64 cross-compilation
toolchain.

Go has supported GOARCH=loong64 as a first-class port since Go 1.21.
The seccomp default profile already includes loong64 support (contrib/seccomp).

Tracked by: https://github.com/containerd/containerd/issues/13641

Signed-off-by: 徐晓伟 <xuxiaowei@xuxiaowei.com.cn>

.github/workflows/ci.yml

@@ -138,6 +138,8 @@ jobs:
             goarch: amd64
           - goos: freebsd
             goarch: arm64
+          - goos: linux
+            goarch: loong64
           - goos: windows
             goarch: arm64
 
@@ -169,6 +171,9 @@ jobs:
           windows/arm64)
             echo "CGO_ENABLED=0" >> $GITHUB_ENV
             ;;
+          linux/loong64)
+            echo "CGO_ENABLED=0" >> $GITHUB_ENV
+            ;;
           esac
 
           if [ -n "${packages}" ]; then

Makefile.linux

@@ -20,7 +20,7 @@ COMMANDS += containerd-shim-runc-v2
 
 # check GOOS for cross compile builds
 ifeq ($(GOOS),linux)
-  ifneq ($(GOARCH),$(filter $(GOARCH),mips mipsle mips64 mips64le ppc64))
+  ifneq ($(GOARCH),$(filter $(GOARCH),mips mipsle mips64 mips64le ppc64 loong64))
     ifeq ($(STATIC),)
 	    GO_GCFLAGS += -buildmode=pie
     endif

RELEASES.md

@@ -237,6 +237,7 @@ verification. Users are free to build from source for their own use.
 | linux/s390x     |  2   | ✅                | ✅             | ❌                    | ✅                    |
 | linux/arm/v7    |  3   | ❌                | ❌             | ❌                    | ✅                    |
 | linux/arm/v5    |  3   | ❌                | ❌             | ❌                    | ✅                    |
+| linux/loong64   |  3   | ❌                | ❌             | ❌                    | ✅                    |
 | darwin/arm64    |  3   | ❌                | ❌             | ❌                    | ✅                    |
 | freebsd/amd64   |  3   | ❌                | ❌             | ❌                    | ✅                    |
 | freebsd/arm64   |  3   | ❌                | ❌             | ❌                    | ✅                    |