aarch64: fix syntax error in asm

The generated code does not restore user sp when returning from
exception handler. That's how a bug hides in plain sight. The assembler
does not even give error!

Signed-off-by: Tianhao <[email protected]>
Suggested-by: CharlyYu <[email protected]>

Author: shrik3

qlib/kernel/arch/aarch64/exception.s

@@ -57,7 +57,7 @@
     ldp x30, x9,  [sp, #16 * 15]
     msr elr_el1, x10
     msr spsr_el1, x11
-.if elx == 0
+.if \elx == 0
     msr sp_el0, x9
 .endif