cpu_linux: Don't spam sched_getcpu failures on qemu
cpu_linux: Don't spam sched_getcpu failures on qemu

__NR_getcpu isn't implemented on qemu, and for some reason
sysconf(_SC_NPROCESSORS_ONLN) returns the number of processers
of the host system, giving a false indication that there is more
than one cpu for the qemu case.

Expand the init_num_cpus sequence to also run sched_getcpu once,
and if that call isn't supported, initialize 'ncpus' to 1.

Later, in gpr_cpu_current_cpu, use gpr_cpu_num_cores to avoid
the system call in cases where we know it isn't supported, or
if the ncpus is otherwise 1.

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