gdb/jit: apply minor cleanup and modernization

gdb/jit: apply minor cleanup and modernization

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

	* jit.c (jit_read_descriptor): Define the descriptor address once,
	use twice.
	(jit_breakpoint_deleted): Move the declaration of the loop variable
	`iter` into the loop header.
	(jit_breakpoint_re_set_internal): Move the declaration of the local
	variable `objf_data` to the first point of definition.
	(jit_event_handler): Move the declaration of local variables
	`code_entry`, `entry_addr`, and `objf` to their first point of use.
	Rename `objf` to `jited`.