Fix compile error of GCC 4

Fix compile error of GCC 4

gcc-4.9.3 doesn't recognize __has_attribute(__fallthrough__) and reports
the following error.

  CC crc/crc32c-arm64.o
  In file included from crc/../os/../file.h:5:0,
                   from crc/../os/os-linux.h:32,
                   from crc/../os/os.h:39,
                   from crc/crc32c-arm64.c:2:
  crc/../os/../compiler/compiler.h:74:20: error: missing binary operator before token "("
  Makefile:501: recipe for target 'crc/crc32c-arm64.o' failed
  make: *** [crc/crc32c-arm64.o] Error 1

Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>