Add debug output for recorded minsyms

Add debug output for recorded minsyms

While discussing this issue:

  https://sourceware.org/ml/gdb-patches/2018-12/threads.html#00082

I added a printf gated by "set debug symtab-create" to be able to
quickly see all minimal symbols recorded by GDB.  I thought it would be
useful to have it built-in, for the future.  Here's how the output
looks:

  Recording minsym:  mst_data                         0x400780    15  _IO_stdin_used
  Recording minsym:  mst_text                         0x400700    13  __libc_csu_init
  Recording minsym:  mst_bss                          0x601058    25  _end

gdb/ChangeLog:

	* minsyms.c (mst_str): New.
	(minimal_symbol_reader::record_full): Add debug output.