gdb/jit: skip jit symbol lookup if already detected the symbols don't exist

gdb/jit: skip jit symbol lookup if already detected the symbols don't exist

To detect whether an objfile is a JITer, we lookup JIT interface
symbols in the objfile.  If an objfile does not have these symbols, we
conclude that it is not a JITer.  An objfile that does not have the
symbols will never have them.  Therefore, once we do a lookup and find
out that the objfile does not have JIT symbols, just set a flag so
that we can skip symbol lookup for that objfile the next time we reset
JIT breakpoints.

gdb/ChangeLog:
2020-07-22  Simon Marchi  <simon.marchi@polymtl.ca>
	    Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* objfiles.h (struct objfile) <skip_jit_symbol_lookup>: New field.
	* jit.c (jit_breakpoint_re_set_internal): Use the
	`skip_jit_symbol_lookup` field.