gas: Don't use frag_align but use plain padding to align .debug_aranges.

gas: Don't use frag_align but use plain padding to align .debug_aranges.

out_debug_aranges uses frag_align to make sure the addresses start
out aligned. Using frag_align will call frag_var[_init], which will
end up calling TC_FRAG_INIT. On arm and aarch64 TC_FRAG_INIT will
generate a $d mapping symbol for the .debug_aranges to show that at
that point a sequence of data items starts.

Such a symbol pointing into a non-allocated debug section will confuse
eu-strip -g. And it seems inefficient and wrong in general to have
additional mapping symbols for debug sections, which won't contain
actual code in the first place.

Just keep track of the aranges header size and use plain padding to
align the addresses which avoids generating any mapping symbols on
aarch64 and arm.

Includes a testcase for aarch64 that PASS with this patch and shows
the extra $d mapping symbol in .debug_aranges before.

gas/ChangeLog

       * dwarf2dbg.c (out_header): Document EXPR->X_add_number value,
       out_debug_aranges depends on it.
       (out_debug_aranges): Track size of header to properly pad header
       for address alignment.

gas/testsuite/ChangeLog

       * gas/aarch64/dwarf.d: New.
       * gas/aarch64/dwarf.s: New.