X86: Don't assert on older Intel CPUs [BZ #20647]

Since the maximum CPUID level of older Intel CPUs is 1, change
handle_intel to return -1, instead of assert, when the maximum
CPUID level is less than 2.

	[BZ #20647]
	* sysdeps/x86/cacheinfo.c (handle_intel): Return -1 if the
	maximum CPUID level is less than 2.