[Darwin, machopic 9/n] Minor code clean-ups.

[Darwin, machopic 9/n] Minor code clean-ups.

Improve some comments, replace some asserts that have been in the code
base for years with checking-asserts.

gcc/ChangeLog:

2019-10-14  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.c: Use unsigned ints for the picbase label
	counters, initialise the vars explicitly.
	(update_pic_label_number_if_needed): Move a variable declaration
	to where it's needed.
	(machopic_output_function_base_name): Use a more strict checking
	assert, and and unsigned int for the picbase label counter.
	(machopic_get_function_picbase): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@276967 138bc75d-0d04-0410-961f-82ee72b054a4