Use ifunc to select the barrier instruction for RDTSC.

This optimizes out runtime switch and removes yet another cpuid from
libc.

Note that this is the first use of ifunc in i386 libc, so
ifunc-capable toolchain is required for building runnable userspace on
i386, same as on amd64.

Discussed with: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week