[ASAN] Use struct instead of array in sancov.py

Summary: When using 32-bit python with 64-bit asan the pc array in sancov.py cannot fit in 64-bit pc's because the type-code 'L' for
arrays in python corresponds to the C type long which is only of 4 bytes. Because of this some of the coverage tool tests fail on
mips. To fix these test possible solutions are to use 64-bit python or use struct.unpack with the 'Q' type-code. We have used
struct.unpack with 'Q' type code since it is not appropriate to have a 64-bit python on all hosts.

Reviewed by kcc, aizatsky

Differential: http://reviews.llvm.org/D18817
llvm-svn: 267126