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 3884bfe9661955543ce203c60f9225bbdf33f6bb), because vfork now don't
use the stack.
diff --git a/libc/arch-arm/syscalls/access.S b/libc/arch-arm/syscalls/access.S
index cf585a3..c0b0333 100644
--- a/libc/arch-arm/syscalls/access.S
+++ b/libc/arch-arm/syscalls/access.S
@@ -3,11 +3,10 @@
 #include <sys/linux-syscalls.h>
 
 ENTRY(access)
-    .save   {r4, r7}
-    stmfd   sp!, {r4, r7}
+    mov     ip, r7
     ldr     r7, =__NR_access
     swi     #0
-    ldmfd   sp!, {r4, r7}
+    mov     r7, ip
     movs    r0, r0
     bxpl    lr
     b       __set_syscall_errno