Don't load undefined bits into rcx before calling the cpuid instruction

The pFeatureC pointer is an uint32_t pointer, therefore only
load 32 bits into ecx.

This avoids loading potentially uninitialized data into the upper
half of the rcx register, fixing valgrind warnings in some build
setups (depending on how the compiler chooses to layout the stack
in the calling function).
Merge pull request #1030 from mstorsjo/cpuid-32bit-param

Don't load undefined bits into rcx before calling the cpuid instruction