Fix clangsupport on FreeBSD.

Summary:
After workaround for https://bugs.kde.org/show_bug.cgi?id=393779 clangsupport plugin
on FreeBSD got broken completely. In https://phabricator.kde.org/D13613 it was agreed
that using cpuid.h header to detect Clang's built-in directory is a bit better hack.

Reviewers: KDevelop

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D14313