cmd/asm,cmd/dist,cmd/go: remove asm -compiling-runtime flag
Currently, dist and go pass a -compiling-runtime flag to asm if
they're compiling a runtime package. However, now that we always pass
the package path to asm, it can make that determination just as well
as its callers can. This CL moves that check into asm and drops the
flag.
This in turn makes dist's copy of IsRuntimePackagePath unnecessary, so
we delete it.
Change-Id: I6ecf2d50b5b83965012af34dbe5f9a973ba0778b
Reviewed-on: https://go-review.googlesource.com/c/go/+/521697
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>