Makefile: fix GO_BUILDMODE setting
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 <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

opencontainers/runc,opencontainers/runc,opencontainers/runc,opencontainers/runc