[asan] abort_on_error=1 by default on Android.

With this change, the default behavior on error is to call abort()
instead of _exit(). This should help the OS to capture a tombstone of
the error.

RAM usage of the lit test suite goes up because of all the tombstone
gathering, so I'm limiting the parallelism of the test target.
Previously it was based on the number of the CPUs on the host
machine, which is definitely wrong.

llvm-svn: 278308