MIPS ptrace build fixes

MIPS ptrace build fixes

Since Pedro's ptrace cleanups, the MIPS buildbot compilation fails.
Code in MIPS native uses ptrace with 3 arguments, where ptrace requires
4.  When looking at the definition of ptrace in
/usr/include/sys/ptrace.h, it shows that it takes a variable number of
arguments.  The wrapper macro in nat/gdb_ptrace.h takes a fixed number
of arguments (4).  That would explain why it used to work and stopped.

I am pushing this as obvious, tell me if there is any problem.

I built-tested this with a MIPS toolchain (ct-ng), but I don't have any
setup to test it.  At least it should put back the buildbot builder in a
better shape.

gdb/ChangeLog:

	* mips-linux-nat.c (write_watchpoint_regs): Add NULL as ptrace's 4th
	parameter.
	(mips_linux_new_thread): Likewise.
	* nat/mips-linux-watch.c (mips_linux_read_watch_registers): Likewise.

gdb/gdbserver/ChangeLog:

	* linux-mips-low.c (mips_linux_prepare_to_resume): Add NULL as
	ptrace's 4th parameter.