target/i386: implement RDPID in TCG

RDPID corresponds to a RDMSR(TSC_AUX); however, it is unprivileged
so for user-mode emulation we must provide the value that the kernel
places in the MSR.  For Linux, it is a combination of the current CPU
and the current NUMA node, both of which can be retrieved with getcpu(2).
Also try sched_getcpu(), which might be there on the BSDs.  If there is
no portable way to retrieve the current CPU id from userspace, return 0.

RDTSCP is reimplemented as RDTSC + RDPID ECX; the differences in terms
of serializability are not relevant to QEMU.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>