MIPS: Avoid overoptimization by GCC.

Without this, it's possible that LTO will discard the calls to
set_except_vector() in the probe for the DADDI overflow bug resulting in a
kernel crash like this:

 [...]
 Mount-cache hash table entries: 256
 Checking for the daddi bug... Integer overflow[#1]:
 Cpu 0
 $ 0   : 0000000000000000 0000000010008ce1 0000000000000001 0000000000000000
 $ 4   : 7fffffffffffedcd ffffffff81410000 0000000000000030 000000000000003f
 [...]

There are other similar places in the kernel so we've just been lucky
that GCC's been tolerant.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -1390,9 +1390,8 @@ unsigned long vi_handlers[64];
 void __init *set_except_vector(int n, void *addr)
 {
 	unsigned long handler = (unsigned long) addr;
-	unsigned long old_handler = exception_handlers[n];
+	unsigned long old_handler = xchg(&exception_handlers[n], handler);
 
-	exception_handlers[n] = handler;
 	if (n == 0 && cpu_has_divec) {
 		unsigned long jump_mask = ~((1 << 28) - 1);
 		u32 *buf = (u32 *)(ebase + 0x200);