cmd/dist,cmd/go: define assembly macros, handle GOARM value with soft/hardfloat

CL 525637 added GOARM_x assembly macros based on GOARM value. But
it did not define the macro in cmd/dist, so the macro is not set
during bootstrapping. This CL defines them.

With CL 514907, cfg.GOARM can also take a soft/hardfloat suffix,
like "7,hardfloat". Handle that case.

For #65601.

Change-Id: I60ffe7e8b623ae693d91d6e8595067a6f76565b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/562995


Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>