Fix procfs.c compilation

Fix procfs.c compilation

procfs.c currently doesn't compile on Solaris:

/vol/src/gnu/gdb/gdb/local/gdb/procfs.c: In function `void _initialize_procfs()':
/vol/src/gnu/gdb/gdb/local/gdb/procfs.c:3734:15: error: invalid initialization of reference of type `const target_info&' from expression of type `procfs_target*'
   add_target (&the_procfs_target);
               ^~~~~~~~~~~~~~~~~~
In file included from /vol/src/gnu/gdb/gdb/local/gdb/inferior.h:40,
                 from /vol/src/gnu/gdb/gdb/local/gdb/procfs.c:24:
/vol/src/gnu/gdb/gdb/local/gdb/target.h:2305:13: note: in passing argument 1 of `void add_target(const target_info&, void (*)(const char*, int), void (*)(cmd_list_element*, completion_tracker&, const char*, const char*))'
 extern void add_target (const target_info &info,
             ^~~~~~~~~~
/vol/src/gnu/gdb/gdb/local/gdb/procfs.c: In member function `virtual char* procfs_target::make_corefile_notes(bfd*, int*)':
/vol/src/gnu/gdb/gdb/local/gdb/procfs.c:3898:16: error: too many arguments to function `gdb::optional<std::vector<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > > > target_read_alloc(target_ops*, target_object, const char*)'
     NULL, &auxv);
                ^
In file included from /vol/src/gnu/gdb/gdb/local/gdb/inferior.h:40,
                 from /vol/src/gnu/gdb/gdb/local/gdb/procfs.c:24:
/vol/src/gnu/gdb/gdb/local/gdb/target.h:341:40: note: declared here
 extern gdb::optional<gdb::byte_vector> target_read_alloc
                                        ^~~~~~~~~~~~~~~~~
/vol/src/gnu/gdb/gdb/local/gdb/procfs.c:3898:16: error: cannot convert `gdb::optional<std::vector<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > > >' to `int' in assignment
     NULL, &auxv);
                ^

Fixed as follows.  Built and ran make check on 64-bit Solaris 11.5/x86
(amd64-pc-solaris2.11) only.

	* procfs.c (_initialize_procfs): Use add_inf_child_target.
	(procfs_target::make_corefile_notes): Adjust to new
	target_read_alloc return type.