rules.h: Add ENV_ macros to detect current architecture

This patch expands the existing ENV_<stage> macros in <rules.h> with a
set of ENV_<arch> macros which can be used to detect which architecture
the current compilation unit is built for. These are more consistent
than compiler-defined macros (like '#ifdef __arm__') and will make it
easier to write small, architecture-dependent differences in common code
(where we currently often use IS_ENABLED(CONFIG_ARCH_...), which is
technically incorrect in a world where every stage can run on a
different architecture, and merely kinda happened to work out for now).

Also remove a vestigal <arch/rules.h> from ARM64 which was no longer
used, and genericise ARM subarchitecture Makefiles a little to make
things like __COREBOOT_ARM_ARCH__ available from all file types
(including .ld).

BUG=None
TEST=Compiled Falco, Blaze, Jerry and Smaug.

Change-Id: Id51aeb290b5c215c653e42a51919d0838e28621f
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: http://review.coreboot.org/12433
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

toolchain.inc

@@ -115,7 +115,7 @@ OBJDUMP_$(1) := $(OBJDUMP_$(2))
 STRIP_$(1) := $(STRIP_$(2))
 READELF_$(1) := $(READELF_$(2))
 CFLAGS_$(1) = $$(CFLAGS_common) $$(CFLAGS_$(2))
-CPPFLAGS_$(1) = $$(CPPFLAGS_common) $$(CPPFLAGS_$(2))
+CPPFLAGS_$(1) = $$(CPPFLAGS_common) $$(CPPFLAGS_$(2)) -D__ARCH_$(2)__
 COMPILER_RT_$(1) := $$(COMPILER_RT_$(2))
 COMPILER_RT_FLAGS_$(1) := $$(COMPILER_RT_FLAGS_$(2))
 LDFLAGS_$(1) = $$(LDFLAGS_common) $$(LDFLAGS_$(2))