Change ch_type to Elf_Word in struct Chdr_data<64>

Change ch_type to Elf_Word in struct Chdr_data<64>

The ch_type field in struct Chdr_data<64>  is 4 bytes, followed by a
4-byte padding.  This change doesn't introduce any functional change
since only the lower 32 bits of the ch_type field are used.

	PR gold/19060
	* elfcpp.h (Chdr::get_ch_type): Change return type to Elf_Word.
	* elfcpp_internal.h (Chdr_data<64>): Change ch_type to 4 bytes
	and add ch_reserved.