libgccjit: Add gcc_jit_global_set_readonly

libgccjit: Add gcc_jit_global_set_readonly

gcc/jit/ChangeLog:

	* docs/topics/compatibility.rst (LIBGCCJIT_ABI_29): New ABI tag.
	* docs/topics/expressions.rst: Document gcc_jit_global_set_readonly.
	* jit-playback.cc (global_new_decl, new_global,
	new_global_initialized): New parameter readonly.
	* jit-playback.h (global_new_decl, new_global,
	new_global_initialized): New parameter readonly.
	* jit-recording.cc (recording::global::replay_into): Use
	m_readonly.
	(recording::global::write_reproducer): Dump reproducer for
	gcc_jit_global_set_readonly.
	* jit-recording.h (get_readonly, set_readonly): New methods.
	(m_readonly): New attribute.
	* libgccjit.cc (gcc_jit_global_set_readonly): New function.
	(gcc_jit_block_add_assignment): Check that we don't assign to a
	readonly variable.
	* libgccjit.h (gcc_jit_global_set_readonly): New function.
	(LIBGCCJIT_HAVE_gcc_jit_global_set_readonly): New define.
	* libgccjit.map: New function.

gcc/testsuite/ChangeLog:

	* jit.dg/all-non-failing-tests.h: Mention test-readonly.c.
	* jit.dg/test-error-assign-readonly.c: New test.
	* jit.dg/test-readonly.c: New test.