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

run_integration_tests.go

@@ -218,6 +218,7 @@ func (env *TravisEnvironment) Prepare() error {
 				"openbsd/386", "openbsd/amd64",
 				"netbsd/386", "netbsd/amd64",
 				"linux/arm", "freebsd/arm",
+				"linux/ppc64le",
 			}
 
 			if os.Getenv("RESTIC_BUILD_SOLARIS") == "0" {