Fix gdb.base/attach-twice.c build on NetBSD

Fix gdb.base/attach-twice.c build on NetBSD

Add a fallback definition of PTRACE_ATTACH that is an alias of
PT_ATTACH. Change the 4th argument of ptrace(2) to 0 as it is
compatible with void * (Linux) and int (NetBSD) arguments.

Include <sys/types.h> for <sys/ptrace.h>.

gdb/testsuite/ChangeLog:

	* gdb.base/attach-twice.c: Include "sys/types.h".
	(PTRACE_ATTACH): Add fallback definition.
	(main): Pass `0' to the 4th argument of `ptrace'.