Only support ARMv6 on ARM platforms

Go 1.21 has switched the default from GOARM=5 to GOARM=7. So far there
have been complaints from Raspberry Pi 1 users, as the first raspberry
pi version only supports ARMv6. Exclude older ARM versions as these are
likely not relevant (rest-server also only supports ARMv6/7) and enforce
the use of software floating point emulation.

changelog/unreleased/issue-4540

@@ -0,0 +1,6 @@
+Change: Require at least ARMv6 for ARM binaries
+
+The official release binaries of restic now require at least ARMv6 support for ARM platforms.
+
+https://github.com/restic/restic/issues/4540
+https://github.com/restic/restic/pull/4542

helpers/build-release-binaries/main.go

@@ -125,6 +125,10 @@ func build(sourceDir, outputDir, goos, goarch string) (filename string) {
 		"GOOS="+goos,
 		"GOARCH="+goarch,
 	)
+	if goarch == "arm" {
+		// the raspberry pi 1 only supports the ARMv6 instruction set
+		c.Env = append(c.Env, "GOARM=6")
+	}
 	verbose("run %v %v in %v", "go", c.Args, c.Dir)
 
 	err := c.Run()