[asan] Read process name from /proc/self/cmdline on Linux.

Rename getBinaryBasename() to getProcessName() and, on Linux,
read it from /proc/self/cmdline instead of /proc/self/exe. The former
can be modified by the process. The main motivation is Android, where
application processes re-write cmdline to a package name. This lets
us setup per-application ASAN_OPTIONS through include=/some/path/%b.

llvm-svn: 243473