[msan][test] Use a large cpusetsize for pthread_getaffinity_np

pthread_getaffinity_np (Linux `kernel/sched/core.c:sched_getaffinity`) fails
with EINVAL if 8*cpusetsize (constant in glibc: 1024) is smaller than
`nr_cpu_ids` (CONFIG_NR_CPUS, which is 2048 for several arch/powerpc/configs
configurations).

The build bot clang-ppc64le-linux-lnt seems to have a larger `nr_cpu_ids`.

Differential Revision: https://reviews.llvm.org/D127368