adjust tracepoint field type based on size

Fix issue #1807

tracepoint may have a format like this:
(from syscalls/sys_enter_socket)
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;

	field:int __syscall_nr;	offset:8;	size:4;	signed:1;
	field:int family;	offset:16;	size:8;	signed:0;
	field:int type;	offset:24;	size:8;	signed:0;
	field:int protocol;	offset:32;	size:8;	signed:0;

Current rewriter generates:
    struct tracepoint__syscalls__sys_enter_socket {
	    u64 __do_not_use__;
	    int __syscall_nr;
	    int family;
	    int type;
	    int protocol;
    };

This is incorrect as in the above structure, offsets of
`family`/`type`/`procotol` becomingg 12/16/20.

This patch fixed the issue by adjusting field type based on its size.
The new structure:
    struct tracepoint__syscalls__sys_enter_socket {
	    u64 __do_not_use__;
	    int __syscall_nr;
	    s64 family;
	    s64 type;
	    s64 protocol;
    };
The offsets of all fields are correct now.

Signed-off-by: Yonghong Song <yhs@fb.com>