Fix and regenerate runtime defs for NetBSD.
Whilst the mcontext struct can be handled across architectures,
the registers are provided as defines that index an array, rather
than as members of the struct. Since these are architecture
dependent, include them via a defs_netbsd_<arch>.go file.
R=golang-dev, m4dh4tt3r, rsc
CC=golang-dev
https://golang.org/cl/6190070
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
