Change how .debug_aranges padding is skipped

Change how .debug_aranges padding is skipped

When GCC emits .debug_aranges, it adds padding to align the contents
to two times the address size.  GCC has done this for many years --
but there is nothing in the DWARF standard that says this should be
done, and LLVM does not seem to add this padding.

It's simple to detect if the padding exists, though: if the contents
of one .debug_aranges CU (excluding the header) are not a multiple of
the alignment that GCC uses, then anything extra must be padding.

This patch changes gdb to correctly read both styles.  It removes the
requirement that the padding bytes be zero, as this seemed
unnecessarily pedantic to me.

gdb/ChangeLog
2021-06-25  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (create_addrmap_from_aranges): Change padding
	logic.

gdb/testsuite/ChangeLog
2021-06-25  Tom Tromey  <tom@tromey.com>

	* lib/gdb.exp (add_gdb_index, ensure_gdb_index): Add "style"
	parameter.
	* gdb.rust/dwindex.exp: New file.
	* gdb.rust/dwindex.rs: New file.