eal: fix %rbx corruption and simplify the code

eal: fix %rbx corruption and simplify the code

Neil Horman reported that on x86-64 the upper half of %rbx would get
clobbered when the code was compiled PIC or PIE, because the
i386-specific code to preserve %ebx was incorrectly compiled.

However, the code is really way more complex than it needs to be.  For
one thing, the CPUID instruction only needs %eax (leaf) and %ecx
(subleaf) as parameters, and since we are testing for bits, we might
as well list the bits explicitly.  Furthermore, we can use an array
rather than doing a switch statement inside a structure.

Reported-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reviewed-by: H. Peter Anvin <hpa@linux.intel.com>