libgccjit: Add option to allow special characters in function names

gcc/jit/ChangeLog:

	* docs/topics/contexts.rst: Add documentation for new option.
	* jit-recording.cc (recording::context::get_str_option): New
	method.
	* jit-recording.h (get_str_option): New method.
	* libgccjit.cc (gcc_jit_context_new_function): Allow special
	characters in function names.
	* libgccjit.h (enum gcc_jit_str_option): New option.

gcc/testsuite/ChangeLog:

	* jit.dg/test-special-chars.c: New test.