add framework for arch-provided makefile fragments, compat source dirs

the immediate usage case for this is to let 32-bit archs moving to
64-bit time_t via symbol redirection pull in wrapper shims that
provide the old symbol names. in the future it may be used for other
types of compatibility-only source files that are not relevant to all
archs.

Makefile

@@ -17,7 +17,7 @@ includedir = $(prefix)/include
 libdir = $(prefix)/lib
 syslibdir = /lib
 
-SRC_DIRS = $(addprefix $(srcdir)/,src/* crt ldso)
+SRC_DIRS = $(addprefix $(srcdir)/,src/* crt ldso $(COMPAT_SRC_DIRS))
 BASE_GLOBS = $(addsuffix /*.c,$(SRC_DIRS))
 ARCH_GLOBS = $(addsuffix /$(ARCH)/*.[csS],$(SRC_DIRS))
 BASE_SRCS = $(sort $(wildcard $(BASE_GLOBS)))
@@ -27,7 +27,7 @@ ARCH_OBJS = $(patsubst $(srcdir)/%,%.o,$(basename $(ARCH_SRCS)))
 REPLACED_OBJS = $(sort $(subst /$(ARCH)/,/,$(ARCH_OBJS)))
 ALL_OBJS = $(addprefix obj/, $(filter-out $(REPLACED_OBJS), $(sort $(BASE_OBJS) $(ARCH_OBJS))))
 
-LIBC_OBJS = $(filter obj/src/%,$(ALL_OBJS))
+LIBC_OBJS = $(filter obj/src/%,$(ALL_OBJS)) $(filter obj/compat/%,$(ALL_OBJS))
 LDSO_OBJS = $(filter obj/ldso/%,$(ALL_OBJS:%.o=%.lo))
 CRT_OBJS = $(filter obj/crt/%,$(ALL_OBJS))
 
@@ -75,6 +75,7 @@ WRAPCC_CLANG = clang
 LDSO_PATHNAME = $(syslibdir)/ld-musl-$(ARCH)$(SUBARCH).so.1
 
 -include config.mak
+-include arch/$(ARCH)/arch.mak
 
 ifeq ($(ARCH),)