Fix gdb build on 32-bit hosts w/ --enable-64-bit-bfd

Fix gdb build on 32-bit hosts w/ --enable-64-bit-bfd

Building for x86_64/-m32 with --enable-64-bit-bfd, compilation fails
with:

 src/gdb/dwarf2read.c: In instantiation of ‘gdb::array_view<const unsigned char> get_gdb_index_contents_from_section(objfile*, T*) [with T = dwarf2_per_objfile]’:
 src/gdb/dwarf2read.c:6266:54:   required from here
 src/gdb/dwarf2read.c:6192:37: error: narrowing conversion of ‘section->dwarf2_section_info::size’ from ‘bfd_size_type {aka long long unsigned int}’ to ‘size_t {aka unsigned int}’ inside { } [-Werror=narrowing]
    return {section->buffer, section->size};
			     ~~~~~~~~~^~~~

This fixes it.

gdb/ChangeLog:
2018-12-07  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (get_gdb_index_contents_from_section): Use
	gdb::make_array_view.