Add support for fchmodat2 syscall

Description:

Running the pacman testsuite under valgrind fails with recent toolchain with:

--701226-- WARNING: unhandled amd64-linux syscall: 452
--701226-- You may be able to write your own handler.
--701226-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--701226-- Nevertheless we consider this a bug.  Please report
--701226-- it at http://valgrind.org/support/bug_reports.html.

This syscall is fchmodat2.

Fix available upstream: https://bugs.kde.org/show_bug.cgi?id=477198

Additional info:

valgrind-3.22.0-2