generate list of crt files to install instead of hard-coding in makefile

this follows the principle of having the source tree layout define
build semantics. it also makes it possible for crt/$(ARCH) to define
additional installable files, which may be needed for midipix and
other future targets.
diff --git a/Makefile b/Makefile
index 3e656d2..ec54880 100644
--- a/Makefile
+++ b/Makefile
@@ -62,7 +62,7 @@
 EMPTY_LIB_NAMES = m rt pthread crypt util xnet resolv dl
 EMPTY_LIBS = $(EMPTY_LIB_NAMES:%=lib/lib%.a)
-CRT_LIBS = lib/crt1.o lib/Scrt1.o lib/rcrt1.o lib/crti.o lib/crtn.o
+CRT_LIBS = $(addprefix lib/,$(notdir $(CRT_OBJS)))
 STATIC_LIBS = lib/libc.a
 TOOL_LIBS = lib/musl-gcc.specs