arm syscall : for eabi call_default don't use stack

In the default case, we don't need to use the stack, we can save r7 with
ip register (that what does eglibc).

This allow to fix vfork data corruption
(see 3884bfe966), because vfork now don't
use the stack.

libc/arch-arm/syscalls/mkdir.S

@@ -3,11 +3,10 @@
 #include <sys/linux-syscalls.h>
 
 ENTRY(mkdir)
-    .save   {r4, r7}
-    stmfd   sp!, {r4, r7}
+    mov     ip, r7
     ldr     r7, =__NR_mkdir
     swi     #0
-    ldmfd   sp!, {r4, r7}
+    mov     r7, ip
     movs    r0, r0
     bxpl    lr
     b       __set_syscall_errno