runtime: make more build-friendly

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
diff --git a/src/pkg/runtime/mcentral.c b/src/pkg/runtime/mcentral.c
index 8463d4e..ff0c2d1 100644
--- a/src/pkg/runtime/mcentral.c
+++ b/src/pkg/runtime/mcentral.c
@@ -15,7 +15,7 @@
 // so that it is faster to move those lists between MCaches and MCentrals.
 
 #include "runtime.h"
-#include "arch.h"
+#include "arch_GOARCH.h"
 #include "malloc.h"
 
 static bool MCentral_Grow(MCentral *c);