fix dubious segment register clear in cpu_init()

Fix a very dubious piece of code in arch/i386/kernel/cpu/common.c:cpu_init(). This clears out %fs and %gs, but clobbers %eax in the process without telling gcc. It turns out that gcc happens to be not using %eax at that point anyway so it doesn't matter much, but it looks like a bomb waiting to go off.

This does end up saving an instruction, because gcc wants %eax==0 for the set_debugreg()s below.

diff -r ced3124154dc arch/i386/kernel/cpu/common.c
@@ -675,7 +675,7 @@ old_gdt:

#endif

	/* Clear %fs and %gs. */

- asm volatile ("xorl %eax, %eax; movl %eax, %fs; movl %eax, %gs");
+ asm volatile ("movl %0, %%fs; movl %0, %%gs" : : "r" (0));

	/* Clear all 6 debug registers: */
	set_debugreg(0, 0);

Added thanks

-Andi