Add a facility for invoking the llvm-config tool when linking a program.

This facility allows LLVMLIBS to be specified with something like:
LLVMLIBS = config --libs jit
instead of:
LLVMLIBS = JIT
with the same effect. However, the llvm-config utility is much more versatile
than the single keyword approach. Note that "config" is the keyword after
which any arguments to llvm-config are allowed. When llvm-config is tested
and working well, we'll start using this and drop support for the JIT
keyword.

llvm-svn: 27057