[HWASan] Ignore shortgranules for global tag selection

Tag selection for global variables is sequential, starting at a
pseduo-ish seed that's based on the hash of the file name.

Previously, it was possible for a global to be assigned a tag in the
range [1,15]. If the global's size was not a multiple of granules (i.e.
`size % 16 != 0`), then the last granule of the global would be assigned
a short granule tag as well.

If the real memory tag of the global (e.g. '04') happened to collide
with the short granule tag (e.g. '04'), then __hwasan_check would
see that the memory tag matched the short granule tag, and dutifully
assume (in this fast check) that everthing is okay.

Unfortunately, if you tried to access the [5,15]th byte, you never get
to the short granule check. This means you miss intra-granule overflows
on the last granule of a global, if said global was assigned a real
memory tag in the range [1,15].

This causes flakiness in certain global tests, if the SHA of the
filename changes between runs.
...