x86: Remove unusual Unicode characters from comments

We've accumulated a few unusual Unicode characters in arch/x86/
over the years, substitute them with their proper ASCII equivalents.

A few of them were a whitespace equivalent: ' ' - the use was harmless.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org