Build lang/go14 without SSE2

This allows to build lang/go14 on old x86 CPUs without SSE2 instructions.
This has an impact on generated machine code performance, but lang/go14
is only useful to boostrap lang/go so it is ok to do so unconditionally.

PR: 230733
Submitted by: Dmitri Goutnik <dg@syrec.org>