Moved Makefile.common to Makefile.rules.  This makes project Makefiles easier
to support, and allows for easier overriding of default configuration values
that used to be in Makefile.common.
Modified Makefile.config.in to determine paths for directories (like
LLVM_SRC_ROOT) and to use the pwd binary as opposed to the shell builtin (this
works better for symbolic links).

llvm-svn: 8377