28a72765 causes following error with GOARCH=arm64 environment.

  # golang.org/x/sys/unix
  asm: InitTextSym double init for "".Syscall
  asm: symbol "".Syscall listed multiple times
  asm: invalid encoding for argument 00004 (/home/junya/go/src/golang.org/x/sys/unix/asm_linux_arm64.s:21)        CALL    runtime.entersyscall(SB)
  asm: invalid encoding for argument 00048 (/home/junya/go/src/golang.org/x/sys/unix/asm_linux_arm64.s:32)        CALL    runtime.exitsyscall(SB)
  asm: makeFuncDebugEntry double process "".Syscall
  asm: assembly failed

Change-Id: Ic7895fecc5f4b1fd6ebef63603c5f720227f5130
Reviewed-on: https://go-review.googlesource.com/86135Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>