checkasm: aarch64: Don't clobber x29 in checkasm_stack_clobber

x29 (FP) is a callee saved register and should be restored on
return. Instead of backing up x29 and restoring it here, back up
sp in a register that we are allowed to overwrite.

This fixes crashes in checkasm on aarch64 since f1b3e131.
For some reason, gcc builds didn't crash, but clang builds do.
Signed-off-by: Martin Storsjö <martin@martin.st>

tests/checkasm/aarch64/checkasm.S

@@ -55,13 +55,13 @@ endconst
 #define CLOBBER_STACK ((8*MAX_ARGS + 15) & ~15)
 
 function checkasm_stack_clobber, export=1
-    mov         x29, sp
+    mov         x3,  sp
     mov         x2,  #CLOBBER_STACK
 1:
     stp         x0,  x1,  [sp, #-16]!
     subs        x2,  x2,  #16
     b.gt        1b
-    mov         sp,  x29
+    mov         sp,  x3
     ret
 endfunc