Fix sancov.py when objdump is llvm-objdump

The sanitizer-coverage.cpp test case was always failing for me. It turns
out the reason for this is that I was building with
-DLLVM_INSTALL_BINUTILS_SYMLINKS=ON and sancov.py's grep regex does not
handle llvm-objdump's disassembly format (hex immediates have a leading "0x").
While touching those lines also change them to use raw string literals since
invalid escape sequnces will become an error in future python versions.
Also simplify the code by using subprocess.check_output() instead of Popen().
This also works with python2.

Fixes https://bugs.llvm.org/show_bug.cgi?id=44504

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D89648