Fill UDSEntry::UDS_CREATION_TIME under linux when glibc >= 2.28

Summary:
The birthtime can be retrieved using using the statx function, available since glibc 2.28. In case the kernel lacks support for the underlying syscall, glibc falls back to stat internally. The validity of the btime field is indicated in the mask field, e.g. when the kernel or the filesystem lacks support for btime.
For details, see http://man7.org/linux/man-pages/man2/statx.2.html

In dolphin, the creation time column is now filled in linux :

FEATURE: 381367
FIXED-IN: 5.58

Test Plan:
Tested using kioslavetest and local build of dolphin.
Tested without HAVE_STATX under linux.

Reviewers: Frameworks, dfaure, fvogt, bruns, broulik

Reviewed By: fvogt, bruns

Subscribers: anthonyfieroni, pino, bcooksley, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20096