vector: add go1.6 build tag for asm code.

Go 1.5 doesn't understand amd64 opcode mnemonics like PADDD or PMULLD.

Change-Id: Ic31ab4d2a8a222ca18288387de40db6ce673d7f1
Reviewed-on: https://go-review.googlesource.com/32132
Reviewed-by: David Crawshaw <crawshaw@golang.org>
4 files changed