libFLAC: Fix cpuid detecton on old Cyrix CPUs

Some old CPUs (eg Cyrix) set EDX but not ECX when executing CPUID.
One of the solutions is to clear ECX before calling cpuid. From
https://bugzilla.mozilla.org/show_bug.cgi?id=1096651#c9 bug

Patch-from: lvqcl <lvqcl.mail@gmail.com>
1 file changed