Collapse the arch,os-specific directories into the main directory
by renaming xxx/foo.c to foo_xxx.c, and so on.

There are no substantial edits here, except to the Makefile.
The assumption is that the Go tool will #define GOOS_darwin
and GOARCH_amd64 and will make any file named something
like signals_darwin.h available as signals_GOOS.h during the
build.  This replaces what used to be done with -I$(GOOS).

There is still work to be done to make runtime build with
standard tools, but this is a big step.  After this we will have
to write a script to generate all the generated files so they
can be checked in (instead of generated during the build).

R=r, iant, r, lucio.dere
CC=golang-dev
https://golang.org/cl/5490053