Add support for ppc64le

This commit adds support for cross-compiling Restic for ppc64le
as part of the release process.

Add a new file:   changelog/unreleased/issue-2277
Modifies:   helpers/build-release-binaries/main.go
Modifies:   run_integration_tests.go

helpers/build-release-binaries/main.go

@@ -251,7 +251,7 @@ func buildTargets(sourceDir, outputDir string, targets map[string][]string) {
 var defaultBuildTargets = map[string][]string{
 	"darwin":  []string{"386", "amd64"},
 	"freebsd": []string{"386", "amd64", "arm"},
-	"linux":   []string{"386", "amd64", "arm", "arm64"},
+	"linux":   []string{"386", "amd64", "arm", "arm64", "ppc64le"},
 	"openbsd": []string{"386", "amd64"},
 	"windows": []string{"386", "amd64"},
 }