cmd/go, cmd/dist: introduce GOMIPS environment variable

cmd/go, cmd/dist: introduce GOMIPS environment variable

GOMIPS is a GOARCH=mips{,le} specific option, for a choice between
hard-float and soft-float. Valid values are 'hardfloat' (default) and
'softfloat'. It is passed to the assembler as
'GOMIPS_{hardfloat,softfloat}'.

Note: GOMIPS will later also be used for a choice of MIPS instruction
set (mips32/mips32r2).

Updates #18162

Change-Id: I35417db8625695f09d6ccc3042431dd2eaa756a6
Reviewed-on: https://go-review.googlesource.com/37954
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>