JIT code debugging doesn't work with dynamically linked LLVM

[Yichao Yu <yyc1992 at gmail dot com>]

The symptom is that when we switch to dynamically linked LLVM (shared
library, instead of static library), the JIT code debugging doesn't
work anymore.

I've just traced down the issue to this piece of code in
`jit_breakpoint_re_set_internal`

```
      reg_symbol = lookup_minimal_symbol_and_objfile (jit_break_name);
      if (reg_symbol.minsym == NULL
      || BMSYMBOL_VALUE_ADDRESS (reg_symbol) == 0)
    return 1;

      desc_symbol = lookup_minimal_symbol (jit_descriptor_name, NULL,
                       reg_symbol.objfile);
      if (desc_symbol.minsym == NULL
      || BMSYMBOL_VALUE_ADDRESS (desc_symbol) == 0)
    return 1;
```

The issue is that `lookup_minimal_symbol_and_objfile (jit_break_name)`
returns the plt entry in our code that links to LLVM so obviously the
lookup for the desc_symbol cannot success since there's no plt entry
for it..... (and even if it does, the breakpoint is still set on the
wrong function)

This can probably be fixed by skipping plt entries in the first symbol
lookup (no patch attached since I don't know how to do that in gdb).
It might also be nice if multiple symbols presented in different
libraries can be supported to.

Yichao Yu