Fix bad asm constrants for x86-64 cpuid()

Bruce reports:

It seems there's a bug in the x86_64 version of do_cpuid() that causes
fio to segfault when built with clang at lower optimization levels:

static inline void do_cpuid(unsigned int *eax, unsigned int *ebx,
                 unsigned int *ecx, unsigned int *edx)
{
     asm volatile("cpuid"
         : "=a" (*eax), "=b" (*ebx), "=r" (*ecx), "=d" (*edx)
         : "0" (*eax), "2" (*ecx)
         : "memory");
}

via Tijl Coosemans:

Should be: "=c" (*ecx)

But you can also use the '+' modifier and remove the input operands:

: "+a" (*eax), "=b" (*ebx), "+c" (*ecx), "=d" (*edx)
:
: "memory"

Signed-off-by: Jens Axboe <[email protected]>

arch/arch-x86_64.h

@@ -5,7 +5,7 @@ static inline void do_cpuid(unsigned int *eax, unsigned int *ebx,
 			    unsigned int *ecx, unsigned int *edx)
 {
 	asm volatile("cpuid"
-		: "=a" (*eax), "=b" (*ebx), "=r" (*ecx), "=d" (*edx)
+		: "=a" (*eax), "=b" (*ebx), "=c" (*ecx), "=d" (*edx)
 		: "0" (*eax), "2" (*ecx)
 		: "memory");
 }