aarch64: improve debuggability on array of enum

aarch64: improve debuggability on array of enum

The current space optmization on enum aarch64_opn_qualifier forced its
encoding using an unsigned char. This "hard-coded" optimization has the
bad consequence of making the array of such enums being completely
unreadable when debugging with GDB because the enum type is lost along
the way.
Keeping this space optimization, and the enum type as well, is possible
when the declaration of the enum is tagged with attribute((packed)).
attribute((packed)) is a GNU extension, and is wrapped in the macro
ATTRIBUTE_PACKED (defined in ansidecl.h), and should be used instead.