Refactor get_syscall_args() and get_error()

Refactor get_syscall_args() and get_error()

* linux/*/get_error.c: Rename all arch-specific implementations
of get_error() to arch_get_error().
* linux/*/get_syscall_args.c: Rename all arch-specific implementations
of get_syscall_args() to arch_get_syscall_args().
* syscall.c (get_syscall_args, get_error): New functions.
(tamper_with_syscall_exiting, get_syscall_result): Do not clear
tcp->u_error before get_error() invocation as the latter does it now.