[lldb] Use local definition of get_cpuid_count

Summary:
This is needed for gcc/cstdlib++ 5.4.0, where __get_cpuid_count is not
defined in cpuid.h.

Reviewers: labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61036

llvm-svn: 359120