Use ctime_r and localtime_r for threadsafety

Use ctime_r and localtime_r for threadsafety

To make these calls threadsafe. localtime_r is provided by gnulib if
necessary, and for ctime_r we can just use it because it is in a linux-
specific file.

gdb/ChangeLog:

2019-11-15  Christian Biesinger  <cbiesinger@google.com>

	* maint.c (scoped_command_stats::print_time): Use localtime_r
	instead of localtime (provided through gnulib if necessary).
	* nat/linux-osdata.c (time_from_time_t): Use ctime_r instead
	of ctime.

Change-Id: I329bbdc39d5b576f51859ba00f1617e024c30cbd