[sanitizer] change the format of coverage dump: instead of always dumping...

[sanitizer] change the format of coverage dump: instead of always dumping 32-bit offsets dump 32-bit offsets on 32-bit arch and 64-bit offsets on 64-bit arch. Also add the 'bits' parameter to sancov.py. This is a user-visible interface change.

llvm-svn: 232555