Use the right name.

libc/sysdeps/linux/arm/Makefile

@@ -25,7 +25,7 @@ include $(TOPDIR)Rules.mak
 LIBC=$(TOPDIR)libc.a
 ASFLAGS=$(CFLAGS)
 
-CRT0=_start.S
+CRT0=crt0.S
 CRT0_OBJ=$(patsubst %.S,%.o, $(CRT0))
 
 SSRC=longjmp.S setjmp.S