On `make clean', kill the core files produced, which are of the form:

On `make clean', kill the core files produced, which are of the form:
core.### where ### is the process ID. We use core.[0-9][0-9]* to avoid killing
core.c, core.cpp, and core.h files which may be part of benchmarks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4913 91177308-0d34-0410-b5e6-96231b3b80d8