Enable 0x10 alignment for aarch64 symbols as is done for x86_64

[Cary Coutant]

> b) this filler section that has the perfect size to make the next real
> section/symbol be 0x10 aligned
>
> u2_.posd shndx:-1, addr:0x40047b  fileoffset:0x47b
> Output_data set_address_and_file_offset      addr:0x40047b  off:0x47b
> data_size:0x5 off:0x47b
> after update off:0x480
>
> where out_aarch64.txt does not have any of these extra padding sections

These are code fill sections. The Target::Target_info structure for
x86_64 sets has_code_fill to true, so that any inserted padding within
a text section gets filled with NOPs. For aarch64, has_code_fill is
set to false, so there are no such sections.

-cary