Fix C++17 array initializer warnings

[malfet]

Fixes warnings like because C++17 is not C99 compatible...

/home/runner/work/cpuinfo/cpuinfo/test/arm-cache.cc:364:5: warning: array designators are a C99 extension [-Wc99-designator]
  364 |                                 [0] = 'P',
      |                                 ^~~
/home/runner/work/cpuinfo/cpuinfo/test/arm-cache.cc:620:5: warning: array designators are a C99 extension [-Wc99-designator]
  620 |                                 [0] = 'P',
      |                                 ^~~
/home/runner/work/cpuinfo/cpuinfo/test/arm-cache.cc:867:5: warning: array designators are a C99 extension [-Wc99-designator]
  867 |                                 [0] = 'P',
      |                                 ^~~
/home/runner/work/cpuinfo/cpuinfo/test/arm-cache.cc:1240:5: warning: array designators are a C99 extension [-Wc99-designator]
 1240 |                                 [0] = 'C',
      |                                 ^~~