[MS] Zero out ECX in __cpuid in intrin.h

Summary:
Some CPUID leafs depend on the value of ECX as well as EAX, but we left
it uninitialized.

Originally reported as https://crbug.com/901547

Reviewers: craig.topper, hans

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54171

llvm-svn: 346265