Automatically cross-test 32bit based on GOARCH

Try to determine the corresponding 32bit architecture from the current
GOARCH and run the tests under that architecture. This only works on a
GOOS/GOARCH that can execute binaries for the smaller architecture, such
as running linux/386 binaries under linux/amd64.

I tested that this works under linux/amd64 and darwin/amd64, the rest of
the architectures is guesswork.

While we still only run regular tests on Intel/Linux architectures, this
covers general integer overflow issues like #629.

Makefile

@@ -13,6 +13,7 @@
 
 GO     ?= GO15VENDOREXPERIMENT=1 go
 GOPATH := $(firstword $(subst :, ,$(shell $(GO) env GOPATH)))
+GOARCH := $(shell $(GO) env GOARCH)
 
 PROMU       ?= $(GOPATH)/bin/promu
 STATICCHECK ?= $(GOPATH)/bin/staticcheck
@@ -37,7 +38,22 @@ else
     test-e2e := skip-test-e2e
 endif
 
-all: format vet staticcheck build test $(test-e2e)
+# 64bit -> 32bit mapping for cross-checking. At least for amd64/386, the 64bit CPU can execute 32bit code but not the other way around, so we don't support cross-testing upwards.
+cross-test = skip-test-32bit
+define goarch_pair
+ifeq ($$(GOARCH),$1)
+	GOARCH_CROSS = $2
+	cross-test = test-32bit
+endif
+endef
+
+# By default, "cross" test with ourselves to cover unknown pairings.
+$(eval $(call goarch_pair,amd64,386))
+$(eval $(call goarch_pair,arm64,arm))
+$(eval $(call goarch_pair,mips64,mips))
+$(eval $(call goarch_pair,mips64el,mipsel))
+
+all: format vet staticcheck build test $(cross-test) $(test-e2e)
 
 style:
 	@echo ">> checking code style"
@@ -47,6 +63,13 @@ test: collector/fixtures/sys/.unpacked
 	@echo ">> running tests"
 	@$(GO) test -short -race $(pkgs)
 
+test-32bit: collector/fixtures/sys/.unpacked
+	@echo ">> running tests in 32-bit mode"
+	@env GOARCH=$(GOARCH_CROSS) $(GO) test $(pkgs)
+
+skip-test-32bit:
+	@echo ">> SKIP running tests in 32-bit mode: not supported on $(GOARCH)"
+
 collector/fixtures/sys/.unpacked: collector/fixtures/sys.ttar
 	./ttar -C collector/fixtures -x -f collector/fixtures/sys.ttar
 	touch $@