build: correct building for ARMv5 and ARMv6

Explicitly set ARM version in GOARM build variable, to avoid relying
on some default value which differs when compiling natively and when
cross-compiling, and which is also incorrectly documented as being
6 when in reality it is 5.

Fix incorrect labelling of ARMv5 builds as ARMv6, and change
architecture of .rpm and .deb packages containing them to
match.

Add ARMv6 builds, to complement existing ARMv5 and ARMv7, and to
reduce disruption due to previous ARMv5 builds incorrectly being
identified as ARMv6, and to provide .rpm and .deb packages with the
same ARMv6 architectures as was previously also published
(then containing ARMv5 binaries).

See #6528

Background info:

https://github.com/golang/go/wiki/GoArm
https://go.dev/doc/install/source#environment
661e931dd1/src/cmd/dist/build.go (L140-L144)
661e931dd1/src/cmd/dist/util.go (L392-L422)

bin/cross-compile.go

@@ -57,6 +57,7 @@ var osarches = []string{
 	"linux/386",
 	"linux/amd64",
 	"linux/arm",
+	"linux/arm-v6",
 	"linux/arm-v7",
 	"linux/arm64",
 	"linux/mips",
@@ -64,10 +65,12 @@ var osarches = []string{
 	"freebsd/386",
 	"freebsd/amd64",
 	"freebsd/arm",
+	"freebsd/arm-v6",
 	"freebsd/arm-v7",
 	"netbsd/386",
 	"netbsd/amd64",
 	"netbsd/arm",
+	"netbsd/arm-v6",
 	"netbsd/arm-v7",
 	"openbsd/386",
 	"openbsd/amd64",
@@ -82,13 +85,16 @@ var archFlags = map[string][]string{
 	"386":    {"GO386=softfloat"},
 	"mips":   {"GOMIPS=softfloat"},
 	"mipsle": {"GOMIPS=softfloat"},
+	"arm":    {"GOARM=5"},
+	"arm-v6": {"GOARM=6"},
 	"arm-v7": {"GOARM=7"},
 }
 
 // Map Go architectures to NFPM architectures
 // Any missing are passed straight through
 var goarchToNfpm = map[string]string{
-	"arm":    "arm6",
+	"arm":    "arm5",
+	"arm-v6": "arm6",
 	"arm-v7": "arm7",
 }