fix build regression for arm pre-v7 from out-of-tree build patch

commit 2f853dd6b9a95d5b13ee8f9df762125e0588df5d failed to replicate
the old makefile logic that caused arch/arm/src/arm/atomics.s to be
built. since this was the only .s file under arch/*/src, rather than
trying to reproduce the old logic, I'm just moving it up a level and
adjusting the glob pattern in the makefile to catch it. eventually
arch/*/src will probably be removed in favor of moving all these files
to appropriate src/*/$(ARCH) locations.

Makefile

@@ -17,7 +17,7 @@ includedir = $(prefix)/include
 libdir = $(prefix)/lib
 syslibdir = /lib
 
-BASE_SRCS = $(sort $(wildcard $(srcdir)/src/*/*.c $(srcdir)/arch/$(ARCH)/src/*.c))
+BASE_SRCS = $(sort $(wildcard $(srcdir)/src/*/*.c $(srcdir)/arch/$(ARCH)/src/*.[csS]))
 BASE_OBJS = $(patsubst $(srcdir)/%,%.o,$(basename $(BASE_SRCS)))
 ARCH_SRCS = $(wildcard $(srcdir)/src/*/$(ARCH)/*.[csS] $(srcdir)/src/*/$(ARCH)$(ASMSUBARCH)/*.sub)
 ARCH_OBJS = $(patsubst $(srcdir)/%,%.o,$(basename $(ARCH_SRCS)))