test: do not run softfloat test with regabiargs

Softfloat mode with register ABI is not implemented yet. In
particular, we did not rewrite the float types in AuxCalls to
integer types, so arguments are still passed in floating point
registers, which do not exist in softfloat mode. To make it work
I think we may want to reorder softfloat pass with expand_calls
pass. We also need to rewrite the OpArgFloatRegs for the spilling
of non-SSA-able arguments, which may involve renumbering interger
arguments. Maybe in softfloat mode we want to just define the
ABI with 0 float registers. They are not fundamentally hard, but
may be not worth doing for the moment, as we don't use softfloat
mode on AMD64 anyway.

Run the test with noregabiargs. Also in the compiler reject
-d=softfloat if regabiargs is enabled.

Change-Id: I8cc0c2cfa88a138bc1338ed8710670245f1bd2cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/308710


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>