linux-user: Emulate CLONE_PIDFD flag in clone()

Add emulation for the CLONE_PIDFD flag of the clone() syscall.
This flag was added in Linux kernel 5.2.

Successfully tested on a x86-64 Linux host with hppa-linux target.
Can be verified by running the testsuite of the qcoro debian package,
which breaks hard and kills the currently logged-in user without this
patch.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>

Message-Id: <Y4XoJCpvUA1JD7Sj@p100>
[lv: define CLONE_PIDFD if it is not]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>