Allow gdb::unique_xmalloc_ptr<T[]>

Allow gdb::unique_xmalloc_ptr<T[]>

Currently, if you try to use the array version of
gdb::unique_xmalloc_ptr (i.e., std::unique_ptr) in order to have
access to operator[], like:

  gdb::unique_xmalloc_ptr<char[]> buf ((char *) xmalloc (10));
  return buf[0];

then the build fails, like:

  /usr/include/c++/5.3.1/bits/unique_ptr.h: In instantiation of ‘std::unique_ptr<_Tp [], _Dp>::~unique_ptr() [with _Tp = char; _Dp = gdb::xfree_deleter<char []>]’:
  main.c:30:61:   required from here
  /usr/include/c++/5.3.1/bits/unique_ptr.h:484:17: error: no match for call to ‘(std::unique_ptr<char [], gdb::xfree_deleter<char []> >::deleter_type {aka gdb::xfree_deleter<char []>}) (char*&)’
      get_deleter()(__ptr);
		   ^
  In file included from src/gdb/common/common-defs.h:92:0,
		   from src/gdb/defs.h:28,
		   from src/gdb/main.c:20:
  src/gdb/common/gdb_unique_ptr.h:34:8: note: candidate: void gdb::xfree_deleter<T>::operator()(T*) const [with T = char []]
     void operator() (T *ptr) const { xfree (ptr); }
	  ^
  src/gdb/common/gdb_unique_ptr.h:34:8: note:   no known conversion for argument 1 from ‘char*’ to ‘char (*)[]’
  Makefile:1911: recipe for target 'main.o' failed
  make: *** [main.o] Error 1

The problem is that we're missing an xfree_deleter specialization for
arrays.

gdb/ChangeLog:
2017-08-10  Pedro Alves  <palves@redhat.com>

	* common/gdb_unique_ptr.h (xfree_deleter<T[]>): Define.