cmake: workaround _num_cores being 0 under qemu emulation

[tserong]

If you're doing something weird like trying to cross compile bits of ceph using qemu emulation, cmake can think there are zero CPU cores. This is because cmake counts "processor" lines in /proc/cpuinfo, which don't exist when running under qemu-aarch64:

  # cat /proc/cpuinfo
  Processor       : ARMv7 Processor rev 5 (v7l)
  BogoMIPS        : 799.53
  Features        : swp half thumb fastmult vfp edsp thumbee neon vfpv3
  CPU implementer : 0x41
  CPU architecture: 7
  CPU variant     : 0x2
  CPU part        : 0xc08
  CPU revision    : 5

  Hardware        : Genesi Efika MX (Smarttop)
  Revision        : 51030
  Serial          : 0000000000000000

This in turn causes heavy_compile_job_pool to not be set, which later makes the build fail, as that pool is referenced in src/tools/ceph-dencoder/CMakeLists.txt. We can work around the problem by setting _num_cores to 1 in this case.

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[jecluis]

looks reasonable to me - I suspect we'll see machines gradually with a higher positive number of cores as we go on, so this should hold for a while.