BACKPORT: arm: new way of handling ERESTART_RESTARTBLOCK

(cherry pick from commit 6b5c8045)

new "syscall start" flag; handled in syscall_trace() by switching
syscall number to that of syscall_restart(2).  Restarts of that
kind (ERESTART_RESTARTBLOCK) are handled by setting that bit;
syscall number is not modified until the actual call.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Bug: 9674955
Bug: 25773781
Change-Id: I340eba00b02a8fef97790365390a8a37ad129839