Makefile: fix GO_BUILDMODE setting

1. Set to empty value by default.

2. Assume Linux (remove GOOS check, since we do not support other OSes).

3. Instead of using a "not-supported" list, use a "supported" list
   (as Go release notes usually say which platforms are supported).
   As of today, -buildmode=pie is supported for:

 * linux/386, linux/amd64, linux/arm, linux/arm64, and linux/ppc64le
   (since Go 1.6, see https://tip.golang.org/doc/go1.6#compiler)

 * linux/s390x (since Go 1.7, which adds the initial port)

 * linux/riscv64 (since Go 1.16, see
   https://tip.golang.org/doc/go1.16#riscv)

   NOTE this does not mean we support these architectures; it is merely
   a way to see if -buildmode=pie can be used.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Makefile

@@ -15,11 +15,11 @@ COMMIT ?= $(shell git describe --dirty --long --always)
 VERSION := $(shell cat ./VERSION)
 LDFLAGS_COMMON := -X main.gitCommit=$(COMMIT) -X main.version=$(VERSION)
 
-ifeq ($(shell $(GO) env GOOS),linux)
-	ifeq (,$(filter $(shell $(GO) env GOARCH),mips mipsle mips64 mips64le ppc64))
-		ifeq (,$(findstring -race,$(EXTRA_FLAGS)))
-			GO_BUILDMODE := "-buildmode=pie"
-		endif
+GO_BUILDMODE :=
+# Enable dynamic PIE executables on supported platforms.
+ifneq (,$(filter $(shell $(GO) env GOARCH),386 amd64 arm arm64 ppc64le riscv64 s390x))
+	ifeq (,$(findstring -race,$(EXTRA_FLAGS)))
+		GO_BUILDMODE := "-buildmode=pie"
 	endif
 endif
 GO_BUILD := $(GO) build -trimpath $(GO_BUILDMODE) $(EXTRA_FLAGS) -tags "$(BUILDTAGS)" \