Reduce packed callstack memory usage

_From [rnk@google.com](https://code.google.com/u/rnk@google.com/) on May 15, 2012 11:35:20_

Thoughts while working on prefix trie: Our packed callstacks can be improved (should file issue). A packed frame is currently 12 bytes:
struct _packed_frame_t {
struct _frame_loc_t {
app_pc addr; # semi redundant with mod+offs, not needed for syscall
const char *syscall_aux; # only needed for syscall, null 99% of the time
} loc;
uint modoffs : 24;
uint modname_idx : 8;
};

Since a syscall frame can really only be in the first frame, it might be possible to move the syscall_aux field into the packed_callstack_t struct.

We could give those extra bits to modoffs + modname_idx and drop the extra full_frame_t indirection. This would even work for x64, since modules larger than 2 GB are hard to create (rip-rel uses 32-bit offset).

The addr field looks redundant with modoffs if you have the base stored somewhere else, but we actually don't have that data in mod_name_t because modules can be reloaded. We'd either have to track all module loads and their bases and use modname_idx to point at that instead.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=887_