rename syscall() macro to ltp_syscall()

This patch renames syscall() macro defined in linux_syscall_numbers.h
to reduce confusion with syscall(2). It also updates all places
where it was previously in use.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>