[dev.typeparams] cmd/compile: make softfloat mode work with register ABI

Previously, softfloat mode does not work with register ABI, mainly
because the compiler doesn't know how to pass floating point
arguments and results. According to the ABI it should be passed in
FP registers, but there isn't any in softfloat mode.

This CL makes it work. When softfloat is used, we define the ABI
as having 0 floating point registers (because there aren't any).
The integer registers are unchanged. So floating point arguments
and results are passed in memory.

Another option is to pass (the bit representation of) floating
point values in integer registers. But this complicates things
because it'd need to reorder integer argument registers.

Change-Id: Ibecbeccb658c10a868fa7f2dcf75138f719cc809
Reviewed-on: https://go-review.googlesource.com/c/go/+/327274


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>