Makefile: respect target os/arch in individual targets
Makefile: respect target os/arch in individual targets

Currently we set GOOS/GOARCH only in host/target targets.
This makes "make fuzzer TARGETARCH=arm64" do a wrong thing,
it still builds for the default value of GOARCH.
Set GOOS/GOARCH for each target individually.

xairy/syzkaller,xairy/syzkaller,dotweiba/syzkaller,zoulasc/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,google/syzkaller,dotweiba/syzkaller,google/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller