libdw: Fix dwarf_macro_getsrcfiles for DWARF 5

libdw: Fix dwarf_macro_getsrcfiles for DWARF 5

Dwarf_Macro_Op_Table::is_64bit conflates the address size and the offset
size: for .debug_macinfo, it is initialized based on the compilation
unit's address size, but for .debug_macro, it is initialized based on
the macro unit's offset size.  is_64bit is used to determine the address
size to pass to __libdw_getsrclines.  For a 64-bit architecture using
DWARF 5 with 32-bit offsets (the common case), this fails because
read_srclines checks that the given address size matches the address
size from the line number program header.

Fix it by splitting is_64bit into separate address_size and offset_size
members.

Fixes: fb90bf3f84b5 ("Support .debug_macro")
Signed-off-by: Omar Sandoval <osandov@fb.com>