Change mode of mksysnum.go to 644.

I also checked all the other files with 755 mode, to verify that no
other Go file has this issue. After this change, only .sh and .pl
files are executable:

  $ git ls-files -s * | grep 100755
  100755 9f73c60660b8978be4cc7041362dc8c220c71bd7 0	plan9/mkall.sh
  100755 85309c4a5ba73b0d05d53c6c262eeaf8d1038f5b 0	plan9/mkerrors.sh
  100755 252e89e14d9cbd083ce61e52f8dd34a62e5fc828 0	plan9/mksyscall.pl
  100755 3c3ab05810ee1203c0d578654dc3069587ba4e94 0	plan9/mksysnum_plan9.sh
  100755 f8dab7511d90d51fbd1470a4b3e718cc559cf7f4 0	unix/mkall.sh
  100755 955dd50f9af178321c881fe1fd5b5f95726dd28c 0	unix/mkerrors.sh
  100755 c44de8d310ef46c084ed38cdaffe74dffbbcbbd5 0	unix/mksyscall_aix_ppc.pl
  100755 53df26bb93acbd6b9841e36ac47ef73f4654b7ad 0	unix/mksyscall_aix_ppc64.pl
  100755 a354df5a6bfd678cb722ccd7ed0795f7c94ac4c2 0	unix/mksyscall_solaris.pl
  100755 20632e14608e37ba2ff7c5b3f467009cf315b51a 0	unix/mksysctl_openbsd.pl
  100755 5453c53b192319c95d47c672a40d7a73f626105f 0	unix/mksysnum_darwin.pl
  100755 6804f41216e732851470c89cb2ae1c552e5515cd 0	unix/mksysnum_dragonfly.pl
  100755 198993d0954d65da7ed143d8edd9bd12117ac697 0	unix/mksysnum_freebsd.pl
  100755 85988b140b9d843c104fe6a2a9f921eab6abcb03 0	unix/mksysnum_netbsd.pl
  100755 84edf60ca12d90b0a6a080947b1a08fb7b6c3b20 0	unix/mksysnum_openbsd.pl

Fixes golang/go#29369

Change-Id: I66eae4cec07806ea6289e229819aef8dbbad9335
Reviewed-on: https://go-review.googlesource.com/c/155417Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>