Replace ARG_MAX with ARG_LAST

Replace ARG_MAX with ARG_LAST

We define an enum ARG_MAX in linux_infcall_mmap, but it is conflict
with macro ARG_MAX which is defined in /usr/include/linux/limits.h.
This causes a build failure below,

 gdb/linux-tdep.c: In function 'linux_infcall_mmap':
 gdb/linux-tdep.c:1945:70: error: expected identifier before numeric constant

the enum in the pre-processed source becomes:

  enum
    {
      ARG_ADDR, ARG_LENGTH, ARG_PROT, ARG_FLAGS, ARG_FD, ARG_OFFSET, 131072
    };

This patch is to replace ARG_MAX with ARG_LAST.

gdb:

2014-12-16  Yao Qi  <yao@codesourcery.com>

	* linux-tdep.c (linux_infcall_mmap): Replace ARG_MAX with
	ARG_LAST.