xtensa: deprecate fast_xtensa and fast_spill_registers syscalls

These syscalls are not used by userspace tools for some time now, and
they have issues when called with invalid arguments. It's not worth
changing signal delivery mechanism as we don't expect any new users for
these syscalls. Let's keep them for backwards compatibility under #ifdef,
disabled by default.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/entry.S

@@ -986,6 +986,8 @@ ENDPROC(fast_syscall_unrecoverable)
  *		j done
  */
 
+#ifdef CONFIG_FAST_SYSCALL_XTENSA
+
 #define TRY								\
 	.section __ex_table, "a";					\
 	.word	66f, 67f;						\
@@ -1049,6 +1051,18 @@ CATCH
 
 ENDPROC(fast_syscall_xtensa)
 
+#else /* CONFIG_FAST_SYSCALL_XTENSA */
+
+ENTRY(fast_syscall_xtensa)
+
+	l32i    a0, a2, PT_AREG0        # restore a0
+	movi	a2, -ENOSYS
+	rfe
+
+ENDPROC(fast_syscall_xtensa)
+
+#endif /* CONFIG_FAST_SYSCALL_XTENSA */
+
 
 /* fast_syscall_spill_registers.
  *
@@ -1064,6 +1078,8 @@ ENDPROC(fast_syscall_xtensa)
  * Note: We assume the stack pointer is EXC_TABLE_KSTK in the fixup handler.
  */
 
+#ifdef CONFIG_FAST_SYSCALL_SPILL_REGISTERS
+
 ENTRY(fast_syscall_spill_registers)
 
 	/* Register a FIXUP handler (pass current wb as a parameter) */
@@ -1398,6 +1414,18 @@ ENTRY(fast_syscall_spill_registers_fixup_return)
 
 ENDPROC(fast_syscall_spill_registers_fixup_return)
 
+#else /* CONFIG_FAST_SYSCALL_SPILL_REGISTERS */
+
+ENTRY(fast_syscall_spill_registers)
+
+	l32i    a0, a2, PT_AREG0        # restore a0
+	movi	a2, -ENOSYS
+	rfe
+
+ENDPROC(fast_syscall_spill_registers)
+
+#endif /* CONFIG_FAST_SYSCALL_SPILL_REGISTERS */
+
 #ifdef CONFIG_MMU
 /*
  * We should never get here. Bail out!