[dev.regabi] cmd/go: workaround -race issue on ppc64le

The race detector on ppc64le corrupts command-line arguments lists if
they contain an empty string, and cmd/go often generates compiler
argument lists containing `-D ""`. Since this is equivalent to not
specifying the `-D` flag at all, just do that. This allows using a
race-detector-enabled cmd/compile on ppc64le.

Updates #43883.

Change-Id: Ifac5cd9a44932129438b9b0b3ecc6101ad3716b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/286173


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>